Concurrency in General
Modern computers use threads to distribute execution between multiple processor cores. There are many reasons for this, including the physics of microchip design and the need for user environments that remain responsive even when one program is performing an intensive computation in the background. Everyone wants to be able to check their email, listen to music, and run their Clojure REPL at the same time! Inside a program, this kind of multitasking can also represent a significant performance gain. While one thread is waiting for data from the disk drive, another for the network, two other threads can be processing data. When done correctly, this can represent a significant gain in performance and overall efficiency. The operative phrase here, though, is "when done correctly." Concurrency can be tricky.
Most computer code is written in a linear manner: do this, do that, then do this. The source code for a method or a function reads from top...
