Focus group is a small group of people (usually 5 to 10) sit around a table and talk about things, like their opinions about products, their past experiences with them, or their reaction to new concepts. Focus groups are good for quickly getting a sampling of users' feelings and opinions about things.
Usability test are about watching one person at a time try to use something (whether it's a Web site, a prototype, or some sketches of a new design) to do typical tasks so you can defect and fix the things that confuse or frustrate them.
The main difference is that in usability tests, you watch people actually use things, instead of just listening to them talk about them. Focus group can be great for determining what your audience wants, needs, and likes - in the abstract.
True things about usability testing:
- If you want a great site, you've got to test. After you've worked on a site for even a few weeks, you can't see it freshly anymore. You know to much. The only way to find out if it really works is to watch other people try to use it.
- Testing one user is 100 percent better than testing none. Testing always works, and even the worst test with the wrong user will show you important things you can do to improve your site.
- Testing one user early in the project is better than testing 50 near the end. A simple test early - while you still have time to use what you learn from it - is almost always more valuable than an elaborate test later.
Based on the Steve Krug's book "Don't make me think"