It can be summarized quickly as: No code gets checked into the main branch unless the changes have been reviewed by a second person.
The practical process is
1) The programmer makes the code changes. These changes may be checked in to a private branch in source control or may be just sitting on his/her hard drive.
2) If the changes are in the 100-line or fewer area (which is a majority of submissions), the programmer goes and "gets a buddy". The buddy is typically a senior level programmer who is already familiar with that code area but that's not a hard rule.
3) The author walks through each line of the changes with the buddy and explains the changes. The buddy points out any stylistic problems, double checks for off-by-zero or null pointer handling code, and reminds the author about any gotchas that may be in that area.
4) If the buddy is satisfied with the code, it gets checked in to the source code control system and the author & buddy's names are noted.
That's it. It adds about 10 minutes to each code submission and keeps a lot of "doh!" errors out of the code. If you don't have a system like this now, I recommend you try it. We find it invaluable.