Hey, Lyla and NocturnalGent. Welcome to DV.

Basically, a WILD is a lucid dream that you go straight into, from the waking state. This usually refers to one that you start at the beginning of your sleep cycle. For this reason, they're usually much harder to do, as you have to get a little further into your sleep cycles to cross into your REM periods, which is when you start dreaming.
WBTB, on the other hand, implies having woken up, sometime during the night, stayed up for a little while while setting the intention on having a lucid dream, and then going back to bed. Doing this, you're going back to sleep, hopefully, within or around one of your REM cycles, which should make recalling the dream and attaining lucidity, much easier.

But Merlock was right. WILD means you flowed into the dream from the waking state. WBTB is just one of the methods for making that happen.