Proftpd is a modular FTP server, which has been around for a long time. Probably the most installed FTP server. If you want a FTP server with the most flexible setup, go with this one.
Step 1: Install proftpd
sudo apt-get install proftpd
Step 2: Configure proftpd on Raspberry Pi
sudo nano /etc/proftpd/proftpd.conf
Step 2.1: Examples of changes
ServerName "RasPiServer"
# Use this to jail all users in their homes
DefaultRoot ~
Port 21
Step 3: Restart proftpd on Raspberry Pi
sudo service proftpd restart