The "mark" facility is one of the most powerful things in Vim. The m[a-z] syntax drops a marker at the current position of the cursor. So you have the full alphabet at your disposal for markers, not just a & b.
The " ' " (the single quote char) with a mark letter is how you address a mark. This can be combined with many Vim commands. Example: drop a marker-h (mh) at the function you are working on, then no matter where you move to in the file, typing "'h" will take you to the "home" function you are working on.
To yank a series of lines, drop a marker-a (ma) at the beginning of the series of lines. Move down to the bottom of where you wish to yank. Hit "y'a" and you will yank all the lines from marker-a to your current position and can then "place" them (I call it drop them) with the p or P commands.
There are plenty of other marker capabilities and many other cool things in Vim. Steve Qualline's great book "VI iMproved", http://www.amazon.com/iMproved-VIM-Landmark-Steve-Oualline/dp/0735710015/ref=pd_bbs_sr_3?ie=UTF8&s=books&qid=1226030977&sr=8-3, is a great read and well worth the money for it. For those that can not afford it, it is also available as an online PDF ftp://ftp.vim.org/pub/vim/doc/book/vimbook-OPL.pdf
