Fundamental Difference is probably the Visibility - GET request is sent via the URL string (appended to the URI with a question-mark as separator), which is visible whereas POST request is encapsulated in the body of the HTTP request and can't be seen.
Length - Since, GET request goes via URL, so it has a limitation for its length. It can't be more than 255 characters long (though this is browser dependent, but usually the max is 255 characters only). Whereas no such maximum length limitation holds for the POST request for the obvious reason that it becomes a part of the body of the HTTP request and there is no size limitation for the body of an HTTP request/response.
Performance - GET request is comparatively faster as it's relatively simpler to create a GET request and the time spent in the encapsulation of the POST request in the HTTP body is saved in this case. In addition, the maximum length restriction facilitates better optimization of GET implementation.
Type of Data - GET request is sent via URL string and as we all know that URL can be text-only, so GET can carry only text data whereas POST has no such restriction and it can carry both text as well as binary data.
Caching/Bookmarking - again for the obvious reason that a GET request is nothing but an URL hence it can be cached as well as Bookmarked. No such luxuries with a POST request.
FORM Default - GET is the default method of the HTML FORM element. To submit a FORM using POST method, we need to specify the method attribute and give it the value "POST".