Most books you see in book stores are published by pubishers, who usually take your work, print it and distribute it, and take their cut of profits.

Alternatively you can decide that you want your work to be self-published, at your own cost, so that you may keep all the profits. Of course this means that you will probably not get a very large distribution...

Basically you simply pay for a printer to print X copies of your work, and thats it, end of story! You now have to go and sell these books to stores or people. Hence the popularity of publishers! :)