Keypress ringtones are sequences of keys you have to press on your cell phone to create a ringtone, ie. you use the included ringtone composer of your mobile. Because you enter those sequences yourself, those ringtones are for free, also referred to as free keypress ringtones.
For each cell phone brand has its own composer, several keypress sequences exist for the same song. For some composers you'll directly find actual key sequences (like: press 111#2238) while sometimes (for some composers or songs) you are given tones and tact (enter c#6 (1/16) etc.).
Another common "text" format is RTTTL (Ring Tone Text Transfer Language). This format consists of three parts: the name of the song, ringtone settings (e.g. octave, default note duration, beat per minute) and the nothes themselves.
So called ringtone converters can ease your ringtone do-it-yourself work. Those programs convert between keypress sequences, RTTTL format and actual ringtone formats (e.g. WAV and formats supported by your cell phone's own composer). You are allowed to listen to the song before uploading to your mobile and best of all you can copy and paste keypress sequences (and RTTTL, etc.) found on the internet and the program will do the rest for you.