Whence you have to download from a server website files the process is called file transfer protocol. FTP stands as abbreviation. In simpler terms it is a network protocol that enables transfer of website files contained in a hosting server to client which may be a folder in your computer or laptop.
An FTP tool usually contains two columns one on the left enables you to direct the files to a folder contained in your computer or lap top. For authentication a dialogue box is provided usually a clear text sign is done. But there can be various options for authenticating the user name and password. Encryption using SSL/TLS is preferred. TLS or transport layer security has preceded SSL or Secure Socket Layer. Symmetric cryptography is used to transmit data over two networked computer the methodology is too complex to be explained here. This offers a private and secure data transmission.
Another protocol used is SSH FTP or SFTP this provides file access, transfer and management over a trusted data stream.
For downloads many FTP software or tools are available on the Internet. The the IP address, username and password is obtained from the hosting service that configure it. The protocol used is configured by obtaining details from the hosting service a well.
Most of the ftp tools are available on the Internet without any charge but the one's meant for highly secured transmission may be available for a price. This applies to both FTP Client and FTP server the later is essential for server management and interact with the former.