Why? As if we can’t just settle with giutars and mandolins, there are octave mandolins, mandolas, citterns, bouzoukis, banjos, etc. etc.
Why?
Do the various instruments have their own merits?
Explain please…
(PS: why do some people spell it mandolin and others mandoline?)
Each of the instruments you mention have their own tone, pitch and tuning styles. I play all of the above and find the difference between them, though sometimes subtle, are right for certain tunes and not for others. I suppose it depends on what you want from each instrument. As for the difference in spelling…I dunno 
I agree with Joseph. The different body shapes and sizes affect the timbre of each instrument. The length of the neck affects the pitch range. The width of the neck affects how you play the instrument. Thickness and composition of the strings also affects the timbre. Some of the instruments you list are double strung (mandolin) versus single strung (bouzouki), so they sound quite different when played. Cittern was originally favoured in England versus the Spanish guitar. It was only in the last century that guitar became more popular.
Then there are the many tonal combinations that can be achieved by mixing these various instruments. The recent release of the Planxty concert on DVD reminds me of how revolutionary their sound was when it first came out; the mixture of mandolin and bouzouki, with a guitar adding bass and backing chords. It was a brilliant idea in itself, but naturally it took the UPs to make their sound seminal. 
djm
Citterns, Irish bouzoukis, and octave mandolins/mandocellos are more or less based around the same thing. The Irish bouzouki differs quite a bit from the Greek instrument from which it takes its name (Alec Finn of De Dannan still apparently prefers to play the original Greek version). An octave mandolin/bouzouki with an additional fifth course of strings is generally termed a cittern, though it too differs a great deal from its historical namesake. I think Stefan Sobell’s website has a little explanation of all the various names that get thrown about. For some reason, it seems like citterns are more popular in Scotland and bouzoukis/octave mandolins are more popular in Ireland.
I have been lucky enough to play with a couple of really top-notch bouzouki players and the sound of one of those things in the hands of someone who knows what they’re doing backing my flute or pipes just made me feel like I could play forever. I’m all for playing unaccompanied too, but sometimes a wee bit of bouzouki can give the tunes (and me) a bit of lift.
Don’t forget the Banjolin! Mandolin strings & scale length with a banjo body!
Banjolin. I nearly forgot about that one. I actually like the sound of it, and have a friend I used to gig with who would break it out every now and then for a lark.
Banjolele 
As to mandolin and mandoline, in the States, anyway, the e-less refers to the musical instrument, and the e’ed refers to a wickedly sharp adjustable kitchen tool intended for slicing veggies very thinly, or not so, but with consistency that most would be hard-pressed to achieve with a knife. Perhaps in Europe the word “mandoline” accounts for both? How about the UK, Ireland, Canada, Australia, or other English-speaking lands?
I play cittern (only as a backup element). If there’s a tune I haven’t learned to play on flute, and it proves easy enough for me to figure out how to do backup for it, it gives me a chance to add a different element to the mix. I like the sound of it, and it can be delicate, or intense if it suits.
I am not familiar with the spelling “mandoline”, and have never seen it used. I can’t speak for the rest of the country, of course.
As to cittern, I would just as soon have a twelve-string acoustic with a cut-away body to access the upper neck (no offense, Nano). But my overall choice would have to be a Gibson SG, 24-fret neck, Pete Rose hardware, and a really bitchin’ distortion box.
djm
No offense taken, djm. I must say, though, that it appears that you’ve been listening to a lot of Steeleye Span lately.
(edited because a “t” mysteriously went missing from my post)
Ah, the T-eater browser virus! Your PC must be infected. 
you’ve been listening to a lot of Steeleye Span lately.
Nope. Actually listening to Zappa and Page. Good Irish b’ys, de pair o’ dem.
djm
djm wrote: "But my overall choice would have to be a Gibson SG, 24-fret neck, Pete Rose hardware, and a really bitchin’ distortion box. "
‘Only a Gibson is good enough…’ words I have lived by for years. I am fondest of their ‘Blues King’ acoustic/electric model, and someday (oh yes) I will own one. Until then, my CL-20 will remain my workhorse, not too big, and a pleasant guitar to play.
The people here refer to mandolines, and quite frankly I don’t known whether it’s really an instrument, so is it really a kitchen appliance? (Sorry, I’m gullible)
And if you don’t wipe yourself with $20 bills, an Epiphone will do. 
I’ll pay 75% less for a finish flaw, but vanity is in the headstock of the beholder.
Don’t forget the 6 string banjo. It’s otherwise known as um… well… a 6 string banjo.
I have one and it sounds like… um… a banjo with 6 strings.
All the best.
Frankly, after what I’ve seen the last few years I think I’d rather have the Epiphone, even if they were the same price. Gibson ain’t what it used to be!
Several years ago I was hanging out in my local guitar shop when I noticed a Gibson Les Paul hanging on the wall near the front counter - it had a price less than half the usual going street price (let alone the MSRP). It looked terrible and I figured it was a used guitar. There were huge runs in the finish all over the guitar. I picked it up and the runs in the finish on the back of the neck ended in little sharp peaks that were high enough that they darned near drew blood when you ran your hand down the neck.
I turned to the owner and said something like, “what darned fool took a spray can to this thing?” That was when he told me that it was a brand new guitar that came that way from Gibson, and that Gibson wouldn’t make good on it. He said when he called Gibson to complain all they would say is “we don’t warrant the finish.”
This thing was so ugly that if I’d been in charge at Gibson I would’ve had somebody on a plane to apologize in person to the shop owner and to make sure the guitar was picked up before any more of the public saw it - it was that bad and that’s not an exageration.
The shop owner finally unloaded the POC by lowering the price to well below his wholesale cost. That was in 96 or thereabouts and he hasn’t carried new Gibson guitars since. Oh, and this isn’t a small shop - it’s an independent store darn near as big as a Guitar Center. I figure that that one guitar, and the shabby way Gibson handled the situation, has probably cost Gibson fifty grand or more over the last few years. That may have been a very isolated case, but I don’t think so from what I’m hearing from people via my website. Let’s just say I’m glad I don’t own stock in them.
Lest people think I’m a Gibson basher, I’ve seen absolute crap out of Fender in the last few years, too. A couple of years ago I worked on a '95 AmStd Strat for a friend of a friend. He said the guitar had never played right and the high E string sounded weak. The bridge was mounted so far off center that, even though when they assembled the guitar they shoved the pickguard as far to the side as possible (until the pickups pressed against the side of the cavity in the body) the strings couldn’t be brought anywhere near the pole pieces of the pickups and the high e string was actually overhanging the edge of the neck where the neck meets the body.
I can’t understand why anyone would buy a Gibson, even if it’s one that happens to have a decent finish, when for about the same price you can get a PRS that’s drop-dead gorgeous and that PRS will stand behind.
If you’re talking acoustics, either Martin or Taylor will play a Gibson into the ground, and both have warranties that they actually honor.
Ukezouki!
Some call it the Pikasso. I call it GUITARZILLA!!!
djm
Oh My Goodness 
Is it safe?
Yikes!What the HECK is that?!? Man! Does it play itself too?
I freely admit that Gibson has done a pretty big nose dive in the quality department as of late…and it is a shame as they used to have, IMO, the best acoustic instruments. I have owned older model axes, and will continue to do so. But there are a couple of newer models which I have picked up, like the ‘Blues King’, that are still worthy of the name Gibson.