What is it?
TEA stands for the Tiny Encryption Algorithm.
http://www.ftp.cl.cam.ac.uk/ftp/papers/djw-rmn/djw-rmn-tea.html
It is a simple/fast cypher that is supposedly quite secure.
Without going into a long dragged out crytptography primer, Im going to leave it to you to research unfamiliar topics. This is in my words, not in that of a professional cryptoanalyst. Feel free to chime in to correct me or elaborate.
Two common modes of cyphers are block cyphers and stream cyphers. I firsted implemented TEA in block cypher mode, first in pure tcl, then in c using critcl. Both worked well, the c version was naturally 100 times faster. The problem with a block cypher is that since you work with blocks of data (ie: 8 bytes), the data needs to be padded to a size divisible by eight, then the padding has to be removed during decryption. Both issues have many common solutions that work well, as did the one I chose.
When sending data over a channel like a socket, the padding becomes a nuisance to handle. A stream cypher works byte by byte, xoring the data byte with a crypted byte based off the key in some fashion. More specifically, CTR or counter mode, is a cypher method whereby you generate the crypted byte by encrypting a 128 bit counter value with your 128 bit key, incrementing the counter after all 8 bytes have been used to encrypt 8 bytes of data. The benifit of this is that you dont have to pad the data, you just discard the unused crypted counter bytes if any are left over.