What are air masses bend at boundaries to form?

The boundaries between air masses are called front. The types of air mass and movements involved determine the type of front.

Warm front: a warm air mass plows into a cold air mass.

Cold front: a cold air mass plows into a warm air mass.

Stationary front: The warm and cold air masses move little relative to one another.

Occluded front: A cold front catches up with a warm front, sending the warm air mass aloft.

Dry line: a dry air mass plows into a moist air mass.