The object is simple: Safely guide your goo balls from one side of the map to the other where a collection pipe awaits. The goo balls can only flow along lattice structures created from the goo balls themselves. Different types of goo balls help create different types of structures. The normal black goo balls create structures similar to rubber. Others exert an outward force that helps them brace against each other and various surfaces. Some hang down like drool. Others create wooden matchstick-like structures.
All of the structures are governed by physics. If a weak structure is built too high without adequate support, it will collapse onto itself, tip over, or break at the weak point. This can be annoying when trying to build tall towers, but it can also be utilized when trying to span chasms. Some levels add other physics elements like wind or water to increase the challenge.
Fresh challenges, interesting music, a sign-driven plot, and a good dose of humor keep you moving from stage to stage until you finally reach the ultimate goal...