The Geek Stuff published part 1 of their introduction to Linux threads

A thread of execution is often regarded as the smallest unit of processing that a scheduler works on.

A process can have multiple threads of execution which are executed asynchronously.

This asynchronous execution brings in the capability of each thread handling a particular work or service independently. Hence multiple threads running in a process handle their services which overall constitutes the complete capability of the process.

  Introduction to Linux Threads Part I