[dlfl]

http://en.wikipedia.org/wiki/MPEG_transport_stream and

http://en.wikipedia.org/wiki/MPEG_program_stream

A TS can contain several program streams:

Transport stream has a concept of programs. Each single program is described by a Program Map Table (PMT) which has a unique PID, and the elementary streams associated with that program have PIDs listed in the PMT. For instance, a transport stream used in digital television might contain three programs, to represent three television channels. Suppose each channel consists of one video stream, one or two audio streams, and any necessary metadata. A receiver wishing to decode a particular "channel" merely has to decode the payloads of each PID associated with its program. It can discard the contents of all other PIDs. A transport stream with more than one program is referred to as MPTS - Multi Program Transport Stream. A single program transport stream is referred to as SPTS.

Also, TS are supposed to have more error correction features, to make them better for transmission by less reliable means.

I'm not sure what practical aspects caused TiVo to use TS in some cases.