A node is remote computer, normally a server but could also be a light or thin client on the blockchain? (network), they are the widely distributed and actually the foundation stones of the technology when using the same , the purpose of these nodes are to relay information to each other, the closest first, then across the entire (blockchain?) network - this information is in the form of maintaining transaction records whatever the type this may be. They also have additional tasks dependent upon their role depending with the cryptocurrency implemented.
As the node owners dedicate computing resources to the network, not only for the relaying of information as above but also to add security and protect against dDOS attacks as well as other exploits such as double spending?, they are incentivized in the form of rewards? for their efforts according to the block reward schedule*