It's very simple actually.
1/ You buy a chapter / campaign. Either separately, in a bundle or whatever special they may have.
2/ Install the game, it will ask you to create an account or use an account that already exists.
3/ When this is done, you're good to go. Just login and start playing.
If you do not have the CD's, or bought the game online, via
www.plaync.com for instance, you can download the Download Client, which will install and download a core amount of files (8000 files or so) which gets to about 750MB on the disk, less when downloading, due to decompression on disk.
You can start playing the game from here, and also either use an existing login or create a login.
There will still be downloads happening in the background, but you can play the game. This is GW's streaming technology at work.
As you play the game and go through the towns / outposts / maps files you may need will be downloaded. These can be from 500-1500 files, depending what area it is.
It is not big though, from 5mb-20mb or so.
Or, if you want everything in one big bang, you can download all the GW files at once, by using the -image switch after gw.exe, i.e. "gw.exe -image"
This is a lot of files though, in the region of 88,000 files and more, after doing the core download. This total of files amounts to about 1.5GB-2GB of data, which will be decompressed on your disk, so eventually after completing everything, your gw.dat file can be in the region of 3.7GB
(I know the totals, cause I had to reinstall on a new laptop yesterday and am now busy doing the -image process. I have all 3 campaigns and the expansion)