Playtype is a danish 'online type foundry' that embodies all that is typography love. They have worked both for commissioned work and type design for sale. Needless to say their fonts for sale are great and a clear example of Danish type design (as themselves state on their website). As type lovers they've made a visual explicative glossary of everything typography- from the different font styles and classifications to other specific concepts as kerning or tracking. Basically, it has everything. Besides of being immensely useful, it is also designed beautifully.
I specially love how they've covered pretty much everything you need to know about typography and always accompanied by a clear, concise visual example. I wish I had this in hand for my typography classes back in school. Here are some screenshots, but just go check the website- there's a lot more!
Brief explanations of the different serif styles with examples
Kerning definition and how it is applied
Explanation of the different cases and type variations