Depending on what you are trying to accomplish, GET and POST are used in different ways.
What is a GET?
In short, a GET is appended to the URL while a POST is embedded in the body of the request. Maybe from time to time, you have noticed something similar to this in your address bar: ?name=something&choice=something-else&category=something-more. This is a GET. Everything after the question mark is a query string. The page that processes this request strips out these values, parses them and acts upon them. The most notable thing here is that your request is public: it appears in the address bar of the browser.
A GET is very simple and easy to implement and parse, but it is 100% not secure. Also, each browser imposes its own limit on the length of an URL. There is no limit specified by a W3C RFC, but it is best to avoid depending on URLs that exceed 255 characters. So, there is a limit to how much data you can pass using a GET. Also you must take care in how you handle special characters. One advantage of a GET is that you could actually save the complete URL, including query string for later retrieval, or you could send that URL to a friend. At times, this can be useful.
What is a POST?
A POST is embedded in the actual body of the message. While not 100% secure, this is more secure than using a GET because the data passed does not appear in the address bar. Also, there is no limit to the amount of data passed. One downside to a POST request is that a page refresh looses the POST data and you cannot save or send a saved URL to a friend.