FTP stands for File Transfer Protocol and is a protocol for transferring files to and from a remote machine usually over port 21. FTP either runs in active or passive mode.
In active mode, the client opens a port on the local machine. Once the client issues a command to transfer a file, the server connects to the port on the client.
In passive mode the client initiates both sides of the connection. When the client issues a command to transfer a file, the client establishes a connection to the server. Passive mode is the most common.
