Probably the simplest example of a deadlock is when a process (A) is waiting on data from process B before it can continue. At the same time process B is waiting on data from process A before it can continue. Both processes are waiting on each other, and neither will ever move forward. Deadlocks generally occur in much more complicated scenarios. Deadlocks occur when multiple processes are waiting for resources (IO ports, data from other processes, etc...) and already have some resources allocated to them that they will not free up until they get the new resources they need. Deadlocks aren't very common at an application level, but OS programmers must be very careful to avoid them.