UML defines a specific notation and set of rules for creating Activity Diagrams. The following are the most commonly used:

• Initial Node - The initial node represents the starting point of the activity diagram.
• Activity Final Node - The activity final node represents the termination point of the activity.
• Action Node - An action node is a type of activity node that represents a single action or behavior of the activity being modeled.
• Activity Edge - An activity edge creates a directed connection between two activity nodes.  It represents the path that a token can take between two activity nodes.
• Decision - A decision has one flow entering and several exiting.  The exiting flows each have a condition that must be met in order to traverse the flow.
• Merge - A merge has several flows entering and one exiting.  The merge denotes that multiple parallel flows are merging at a single point.  Only one flow must reach the merge point in order to continue to traverse the flow to the next activity.
• Fork - A fork has one flow entering and several exiting.  A fork denotes that several processes are occurring in parallel.
• Join - A join has several flows entering it and one exiting it.  A join denotes that multiple parallel flows are merging at a single point.  All flows going into the join must be completed before the next activity can start.

There are others, but these eight symbols constitute the basic notation used by nearly every Activity Diagram.