The Livepeer Protocol specification lays out the various roles in the Livepeer Network, the various transactions types supported, the cryptoeconomic security measures in place to prevent collusion, and the token distribution mechanics. As a high level summary, users that participate in the Livepeer protocol have one of following roles:
An orchestrator is a protocol-aware, smart, 24/7 process that is responsible to the end user of the network for transcoding jobs being performed correctly. They stake LPT to secure the work that they perform, and ensure it is done correctly. They can be penalized if they maliciously cheat and mistranscode an end users content.
A transcoder on the other hand, is a simple process that knows how to take an input segment of video, and transcode it to the desired outcome. It is not Livepeer protocol aware, it has no requirement for high reliability or being online 24/7, it makes no representation to the end user, and it has nothing at risk. A user in this role can run many transcoder processes, likely connected to many GPUs. It is possible to come up with constructions for "public transcoder pools" that allow orchestrators to distribute work to random transcoders.
A broadcaster is a protocol-aware process that fulfills the demand side of the Livepeer network, it takes input streams from the end-user on its exposed RTMP interface to have them transcoded by the infrastructure providers running on Livepeer. The broadcaster takes care of splitting up streams into segments for transcoding and aggregating the transcoded results in a media playlist. Broadcasters are able to determine the output renditions and maximum price per pixel for transcoding jobs it sends into the Livepeer network and pay for these jobs in ETH using probabilistic micropayments.
A delegator is a Livepeer tokenholder who participates in the network by "staking" its tokens towards orchestrators it believes are doing good and honest work. You can think about staking like putting a deposit down. When a broadcaster pays fees into the network, both orchestrators and delegators earn a portion of those fees as a reward for ensuring a high-quality and secure network.
The Livepeer core team along with the broader Livepeer community drives development of the protocol and ecosystem.