Every player that is part of a factions other than the Fallacy Fairies starts with a Cavewall Uplink (also sometimes referred to as Cavelink, Cufflink, Cuff, Cuplink, or CUP). Aesthetically it is different for the different factions, but the function is the same. Fallacy Fairies instead start with a Leafwhisper.
The Cavelink serves many functions. It is the style in which menu GUI appears, and is the connection to your Matrix.
The Humanist faction has one with an appearance like a high resolution digital lenses that drops down like glasses.
The Reaper faction has one with the appearance of a green dot-matrix screen.