In this two part series about FTP we are going to look at what an FTP server is and what it is used for. The first part will cover the theory and the second part will put all the theory into practice; it's where we will actually implement an FTP server. I'm doing the theory first to give you a bit of background about the FTP protocol. This will make it easy for you to understand the purpose of each command in the protocol as well as how to implement them in the next article.
FTP Server: The Theory - FTP Commands (Page 3 of 5 )
The FTP commands are at the heart of the FTP protocol. It is essentially the language of communication between the client and the server. Below, I’ve provided a full list of commands used in the protocol:
USER <SP> <username> <CRLF>
PASS <SP> <password> <CRLF>
ACCT <SP> <account-information> <CRLF>
CWD <SP> <pathname> <CRLF>
CDUP <CRLF>
SMNT <SP> <pathname> <CRLF>
QUIT <CRLF>
REIN <CRLF>
PORT <SP> <host-port> <CRLF>
PASV <CRLF>
TYPE <SP> <type-code> <CRLF>
STRU <SP> <structure-code> <CRLF>
MODE <SP> <mode-code> <CRLF>
RETR <SP> <pathname> <CRLF>
STOR <SP> <pathname> <CRLF>
STOU <CRLF>
APPE <SP> <pathname> <CRLF>
ALLO <SP> <decimal-integer>
[<SP> R <SP> <decimal-integer>] <CRLF>
REST <SP> <marker> <CRLF>
RNFR <SP> <pathname> <CRLF>
RNTO <SP> <pathname> <CRLF>
ABOR <CRLF>
DELE <SP> <pathname> <CRLF>
RMD <SP> <pathname> <CRLF>
MKD <SP> <pathname> <CRLF>
PWD <CRLF>
LIST [<SP> <pathname>] <CRLF>
NLST [<SP> <pathname>] <CRLF>
SITE <SP> <string> <CRLF>
SYST <CRLF>
STAT [<SP> <pathname>] <CRLF>
HELP [<SP> <string>] <CRLF>
NOOP <CRLF>
You can implement as many or as few commands as you want in your custom FTP server application. Of course, the more commands you implement the more robust your application is going to be. The FTP protocol is under constant review, so try to keep a close eye on the changes that are being made.
Delphi-Kylix 
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Building An FTP Client With PHP
