Top Down View (orthographic)
My Free Tile Sets
Angled View (isometric)
These are multiple images that have been put together to create themed environments which are set up so they can be used together in many different ways. Typically they are tiled squares of the same image and size, but in different variations. Sizes can vary depending on the scale size of the game being developed, some are 8 x 8 pixels, 16 x 16 pixels, and 32 x 32 pixels which generally seem the most popular, but I don't believe there is a written rule to what they should be, it really is dependent on what the developer wants from a tile set.
The different variations I mentioned could be small changes, such as a platforms edge, or change in scenery, although coming from the same style tile, this allows the same tile to be used over and over without using massive amounts of memory and with each varied tile it breaks up the monotony, keeping it consistent looking.
Tile Sets don't just have to be used for generating the platform setting on your game level either, they can be used for backgrounds too, and in some cases they can be even used as an object sprite in your games. Used in this way altogether it allows level creation to be much simpler, it's much more easier and quicker to place a tile then it is to draw each level from scratch.