Contexts in GTD

GTD is all about focus and clean mind. Contexts are there to help you stay focused when you work through your "DO" list. Here's how.

Any place, any time vs. Exact place, exact time

There are a few types of activity that can be performed in any place, any time, with any tools equally well. Like, "Think about ideas for my GF's birthday present". You can think pretty much everywhere: at the office, in your car, at the restaurant etc.
There are much more other tasks that you can only perform when you are in a certain place, time or seeing certain people. "Replace a lightbulb in the garage". You can only do that when you're in your garage. Or, "Take a photo of a sunrise". Obviously, you cannot do that in the afternoon. "Reply to Joe's email". You'll need a PC or a mobile device to do that. You got the point.

Meet contexts

Now, each of this conditions is called a context. You got a context-free task? Fine, you'll be able to complete it wherever you are. Need to talk to your mom who lives 150 miles away? Well, you would want to place it into "Phone" context. Need to buy more file folders? A good candidate for a "Department store" context.
Frankly, the "context-free" example of thinking about present for GF is best done in a "sheet of paper" context as it's more a brainstorming and requires capturing your thoughts on paper.

Now, if you would try working on all these tasks without paying attention to context, you'd be lost. "Replace a lightbulb"? OK, I go outside to my garage. "Call Kate"? Let's go to my computer. "Buy milk"? Oh, need to drive to the grocery... As you can see, this way you'd not be quite productive.

So how it works?

How this works in terms of GTD? When you process your inbox, you assign a context to each task. Afterwards, when you're in a certain context, you pull out the list of tasks you have for that context and work on these tasks. Keep working as soon as you remain within the suitable context.
When it's time to switch to another context, you're now in a good position to start working on that context tasks. Easy enough. All it takes is to assign context to your tasks as you process them.

That's it. Hope that helps.