Our wiki has two main sections: Learn (with theoretical information) and Guides (with practical instructions for working with parachain)
What is a parachain?
A parachain is an application-specific data structure that is globally coherent and validatable by the validators of the Relay Chain. Most commonly a parachain will take the form of a blockchain, but there is no specific need for them to be actual blockchains. They take their name from the concept of parallelized chains that run parallel to the Relay Chain. Due to their parallel nature, they are able to parallelize transaction processing and achieve scalability of the Polkadot system. They share in the security of the entire network and can communicate with other parachains through XCMP.
Parachains are maintained by a network maintainer known as a collator. The role of the collator node is to maintain a full-node of the parachain, retain all necessary information of the parachain, and produce new block candidates to pass to the Relay Chain validators for verification and inclusion in the shared state of Polkadot. The incentivization of a collator node is an implementation detail of the parachain. They are not required to be staked on the Relay Chain or own DOT tokens unless stipulated to do so by the parachain implementation.