How can I connect to ftp server in terminal?
ftp hostname


or

ftp://username:password@hostname


Now, once connected the 5 most common options are:

cd foldername      #e.g. cd /downloads/recent
get filename       #e.g. get thisisthefileiwant.text
put filename       #e.g. put sendthistotheserver.txt
mget filenameregex #e.g mget * to get every file in a directory
mput filemameregex #e.g. mput *.txt to upload every txt file to the server


The only trickery is sometimes you might need to put it into binary mode by typing binary

To exit, type bye

The difference between mine and @grgarside 's solution is his forks it to the Finder using open, while mine uses Terminal