As the name implies, the main feature of RPG Maker is the ability to design your own RPG. It allows you to create a Japanese-style RPG, similar to the older Final Fantasy games. You supply the story, dialogue, create the maps and dungeons, and set the music. Really, it's all the fun parts of designing a game, without the hassle of having to program the technology behind it. If you choose to, you can create your own sprites and art as well. RPG Maker allows you to import spritesheets and tilesets, as well as your own MIDI music or sound effects.
Most of your time will be spent in the main editor window, where you design the dungeon maps and place objects and events. If you can use Microsoft Paint, you can use the map editor in RPG Maker. You simply select the tile you want to use, and then place it wherever you want on the screen. You can also fill areas or create shapes with the tiles. There are a few layers that you can switch between to create more complex map scenery as well.
Events are the main driving force behind the RPGs you will make. To create an event, you open up the event window and select from a list of commands that you want the event to perform. You can use any number and combination of these commands to create complex events, such as character interaction, or something as simple as a treasure chest for the players to open. More advanced users can add their own scripts to RPG Maker XP and later versions, using the Ruby programming language.