And just in case you're not 100% sure yourself, here is Steve's 30-second elevator pitch...
"Usability tests are about watching people actually try to use what we’re building, so we can detect and fix the parts that confuse or frustrate them.
Focus groups are about having people talk about things, like their opinions about our products, their past experiences with them, or their reactions to new ideas that we show them.
So the main difference is that in usability tests, you watch people actually use things, instead of just talk about them."
You say “potato,” I say “focus group” - blog post by Steve Krug