It is an executable file —
distrib.exe. Distribution agent is responsible for moving snapshot and transactions (for Transactional Replication) to the Subscribers.
When we configured replication, it creates an agent and its profiles get created. Generally, an agent profile has a set of parameters, that are queried each time an agent runs and logs into the Distributor. Apart from agents, it creates some maintenance jobs to keep the database synchronized and in shape. Refer to the following screenshot: