I don't think I've made much of a comment on self-publishing here, so thought it worth a mention. If you want to discover some of the pitfalls, I can't do better than refer you to the hyper-knowledgeable Lynn Price's blog. Lynn is the editorial director of a US publisher, and has, shall we say, very clear ideas on self-publishing.
I'll consider four broad scenarios why someone might be interested in self-publishing, and what I'd suggest. The first is someone has written something they'd like to see in print, but is well aware it's never going to be a commercial book. The history of your family, for instance. Here I'd recommend a service like Lulu. You upload your book (this is a bit fiddly, but not too hard) and they will produce one or more copies as and when you like at quite a reasonable price. The end result looks like a real book, though the covers are slightly flimsy compared to a typical commercial press.
The second possibility is that you have written, say, a novel that you have shopped around the publishers and agents, and you know it isn't going anywhere, but you are very fond of it and want friends and relations to be able to read it comfortably, rather than as a pile of paper. Again, Lulu offers a good solution. I'd give as a great example of this Henry Gee's book By the Sea, which he has accepted is unlikely ever to be published, but I and quite a few others have bought in the Lulu form and enjoyed reading.
The third possibility is that you have written what you believe is a great novel and you have heard about all these people who get their self-published novel given huge advances by publishers when they see how well it's doing. So you go to one of the many publishers advertising online who say that they will take your book, turn it into something wonderful and use their marketing expertise to get it into all the bookshops.
What these websites won't say is that they will charge you a fee - usually thousands of pounds - to do this. Nor will they mention that they won't properly edit the book, will do practically no marketing, and their books rarely get into shops at all. Sure, they'll be listed on Amazon - but anyone can get a book listed on Amazon. These companies are known as vanity publishers. That makes it sound like the author's vain, but what such 'publishers' do is take innocent people's money, pretend they are fulfilling their dreams - and then leave them with a hole in their bank balance and nothing to show for it.
The answer for these people is, don't do it. If you want your book properly published, get taken on by an agent who will submit you to real publishers. If you just want to get into print, try Lulu.
Finally there are people who need lots of copies of a book for their work. Perhaps they are trainers, and have written a book that they want to use in training. I would encourage such people to try to get in published first. With a book like this you can submit direct to publishers. If you do get it taken on, you won't have to pay for publication, and your book will have more kudos. Alternatively there are plenty of respectable publishers who don't claim to turn your work into a 'real book' but will produce copies for you to order. If you are ordering in bulk, you will probably be able to beat Lulu for pricing, and get a better quality of product. I can't recommend specific publishers - look for recommendations from good sources like the Society of Authors.