Nodes are the active electronic devices (may be computers, laptops, phones etc) that are connected to blockchain network. They are part of the widespread network of blockchain.
In general the role of a node is to maintain, support and process the trustworthy information (or) transaction (or) services of the blockchain network. Those need to be in line with the predefined consensus protocol of the distributed ledger.
A specific node is a node that possesses the same responsibility as nodes do but with an additional important activity called mining.