A stream is an abstract interface for working with streaming data. It allows you to read data from a source or write data to a destination in a continuous fashion. Streams are a powerful way to handle large amounts of data efficiently, as they allow you to process data as it is being produced or consumed, rather than waiting for all the data to be available before beginning processing.

There are four types of streams in Node.js:

• Readable streams: These streams allow you to read data from a source. Examples of readable streams include fs.createReadStream (used for reading from a file), process.stdin (used for reading from the standard input), and http.IncomingMessage (used for reading the body of an HTTP request).

• Writable streams: These streams allow you to write data to a destination. Examples of writable streams include fs.createWriteStream (used for writing to a file), process.stdout (used for writing to the standard output), and http.ServerResponse (used for writing the body of an HTTP response).

• Duplex streams: These streams are both readable and writable. An example of a duplex stream is a socket connection, which can be used to send and receive data.

• Transform streams: These streams are duplex streams that can modify or transform the data as it is being written to or read from. An example of a transform stream is the zlib.createGzip stream, which compresses data using the gzip algorithm as it is written and decompresses it as it is read.