A Focus Group is a bunch of people chatting about how the feel about something. How they feel has nothing to do with if it works or if they will buy it. But you’ll get some good ideas.
Usability Testing tells you if a thing, usually a web site, lets users get a job done, such as buying something from you. It shows that all your opinions about your site don’t matter at all — it’s what the customer does with it.
Analytics tell you afterward what really happened, but not necessarily why.
Emissions Testing tells you if a marketer is blowing smoke out of their tailpipe.
Many marketing execs don’t know the difference in these types of testing (or they rely on the one that they are comfortable with, even if it’s the wrong one).