A cell is a fixed-length packet used in ATM High-speed internet technology. To be specific, these packets are 53 bytes of which 48 is the payload*. The beauty of these fixed-length packets is that the headers do not need to contain size information, nor does an ATM Switch need to check the size of a packet to make sure it received the whole thing. This allows the ATM Switch to rapidly accept a cell and almost immediately send it back out again. This is in contrast to a router which has to check the size of a packet to know when to stop receiving and when to start sending. This is very fast.
*Why 48 bytes? The story I've heard, and believe, is that two different
standards organizations each wanted the payload to be a different sizes. One wanted 32, the other wanted 64 -- they
compromised.