BPEL processes have a series of steps and each step is an activity. BPEL process logic is performed by the activities. For clients, BPEL processes looks like any other web service. In other words, a BPEL process is a web service that coordinates, integrates, and orchestrates the basic web services to serve a business function. OASIS WS-BPEL specifications classify activities in the following categories:
Basic activities: Enables the process behavior such as invoking and performing web services operations, updating variables, and partner links
Structured activities: Enables the control flow logic such as flow control activities