FTP Protocol Overview:

FTP is the abbreviation of The File Transfer Protocol. FTP is documented in RFC 959. FTP operates over TCP connections, usually to port 20(Data)/21(Control). During an FTP session there will be two separate network connections: one between the PIs and one between the DTPs. The connection between the PIs is known as the control connection. The connection between the DTPs is known as the data connection. 

There are two kinds of FTP, active FTP and passive FTP. With normal or active FTP, the control connection is initiated by the client side and the data connection is initiated by the server side. (Active mode is also sometimes called port mode). The other option is passive FTP, in which case the client side establishes the data connection.

As FTP sends the username and password as clear text, it's insecure. Both SCP and SFTP are similar in function to FTP but use Secure Shell (SSH) authentication and encryption. 

References:

Analysis of the File Transfer Protocol (FTP)
    Briefly describe FTP protocol processes, detailed steps of active vs. passive FTP.

Active FTP vs. Passive FTP, a Definitive Explanation

The FTP and Your Firewall / NAT / Load-Balancing Router
    Describe some problems of FTP, like: why port poses problems for routing devices, why PASV poses problems for firewalls, why PASV poses problems for FTP servers on internal networks, why PASV poses problems for FTP servers behind load-balancing routers, deadlock - when there are restrictive firewalls on both sides, problems when the FTP server is listening on a non-standard port number, problems caused by the firewall prematurely timing out a valid FTP session.