This is a very common question in the WordPress universe for those that are just starting out. It may seem at first glance that they are the same (and they do have some similarities) but they serve different purposes.
Posts can be thought of as a blog. Most often they are used specifically for that purpose. However, even when they're not directly contributing to a blog, posts will read like blogs in that they will list in reverse chronological order, with a date and time visible, and can be categorized and archived. E.g. Blogs, press releases, member spotlights.
For more information on creating Posts, check out the article here.
Pages are the static interior of your site. They do not have the ability to be categorized or tagged and they do not have a visible date and time of posting. Pages can also be grouped together so that one page is a "parent" to another. E.g. About Us, What We Do.
For more information on creating Pages, click here.