Pop and lock is a type of hip hop/breakdancing. It is often called popping n locking or poplocking and may simply be called popping for short. As with many dance styles, it is somewhat hard to define and even harder to describe through writing, but I'll give it a try.
A little background:
Popping is technically a name for a different style of dance that was later assimilated into poppin' n lockin'. It is a type of funk dance adapted for street dancing that involves rhythmic, jerky movements in time to the music. Certain aspects of it might remind you of the robot, but it's much, much cooler than that. The beat of the music can be seen taking over various parts of the dancer's body and jerking them in sudden, unexpected ways. It is somewhat influenced by locking.
Locking is also a style of funk/street dance, and is a lot closer to what most people think of when they think funk. It involves a lot of distinct, fast-moving arm and hand movements, and in its modern form it looks a lot like krumping, although it is a little more stylized and a little less violent in its movements. It is actually a odd offshoot from the Funky Chicken dance, but don't let that fool you; it's well worth watching a good locker do his thing.
Pop and lock combines these two in an interesting way. The dancers alternate between jerky but precise, often seizure-like movements and sudden fluid movements. The beat of the music may seem to pulse through the dancers, moving their bodies into odd and awkward postures. Pop and lock dancing is often done to techno/dance/trance music, but it is still closely associated with hip-hop and b-boying.
Experienced dancers often undertake feats of endurance that rival anything you'll see b-boys do, although while breakdancers tend to aim for flashy athletic moves pop and lock tends to aim for freaky and often humorous effects. A pop and locker is often more a contortionist than an athlete. In addition to jerky, strobe-like movements, dancers often restrict movement to certain body parts or muscle groups, and often spend part of the dance on the floor. The locking influence often makes these dances extremely energetic, rivaling jumpsyle for sheer exhaustion through bounce. While there is no dress code, old loose suits and retro hats are common -- kindov like the 1980s version of zoot suiters.
In recent years shows like So You Think You Can Dance and video sites like YouTube.com have been bringing pop and lock dancing into the public eye. It is also apparently very popular in Asia.
If you'd like to see some videos, YouTube is full of them. I recommend http://www.youtube.com/watch?v=WPeLea4AVZ0 (a little more on the popping end of the spectrum) and http://www.youtube.com/watch?v=oblRnGLLMyk&feature (a little more on the locking end of the spectrum).