What do a t-shirt and a computer program have in common? They are both composed of many threads! While the threads in a t-shirt hold the shirt together, the threads of a computer program alllow the program to execute sequential actions or many actions at once. Each thread in a program identifies a process that runs when the program asks it to (unlike when you ask your roommate to do the dishes).
Threads are typically given a certain priority, meaning some threads take precedence over others. Once the CPU is finished processing one thread, it can run the next thread waiting in line. However, it's not like the thread has to wait in line at the checkout counter at Target the Saturday before Christmas. Threads seldom have to wait more than a few milliseconds before they run. Computer programs that implement "multi-threading" can execute multiple threads at once. Most modern operating systems support multi-threading at the system level, meaning when one program tries to take up all your CPU resources, you can still switch to other programs and force the CPU-hogging program to share the processor a little bit.
The term "thread" can also refer to a series of related postings in an online discussion. Web-based bulletin boards are made up of many topics, or threads. The replies posted in response to the original posting are all part of the same thread. In e-mail, a thread can refer to a series of replies back and forth pertaining a certain message.