I once used pplive but have not used this kind of software for a few years.
When you use this type of software to watch TV/Video or to download files, your computer particiates as part of a P2P network. In other words, when you are using it, you are also sharing the resources (CPU, memory etc) of your PC with others who are also using the network.
Here are two links (from wiki) about P2P:
In English: Peer-to-peer
點對點技術