How does a glider thermal?

Hello,
I just watched a movie online (Youtube) about gliders and saw one
thermaling and gaining altitude. How is this is done? I am a presolo
power pilot (Just finished with my 12th hour) and have been taugt
that a power plane will climb when excess power is availible.
(Steepen the angle of attack, and the airplane will start climbing
steady when all 4 forces are equal) How does a glider do that, since
there is no engine? Does the thermal change the relative wind?
(Which can\'t because the airplane creates the relative wind as it
flys through the air, right?)

Really interesting stuff.