Usability testing is focused on the individual experience of one user. A focus group is a collective testing effort wherein the opinions and experiences of the individual group members shape the opinions and experiences the rest.
Focus groups are best used early on when the site is still in the stages of identifying its audience. Usability testing should be used throughout the process.
The best kept secret is you can use anyone for it because, most of the time, all they need are basic web skills.