Lava domes are basically what the name describes, domes of lava. They are formed through accumulations of thick, highly viscous (a high viscosity refers to how fast a liquid can flow- example: syrup has a high viscosity while water has a low viscosity) lava that soon cools and hardens into a shell of rock. If conditions are right, say more lava rises up or the environment is too dry, the rock shatters and tumbles down the sides. The process repeats itself and after a while the dome builds itself up.
Usually, two types of lava domes form. Depending on the type of lava that flows out, short and steep "coulees" can form or, in another type, lumpy protrusions form around the lava vent. Lava domes are often found growing inside the craters of composite Volcanoes such as the Katmai Volcano of Alaska.
Lava Domes 
