Roll20 is oriented towards in-person gaming over the internet. However the fact it is a internet application means you can play with anybody in the world whose schedule matches yours.
Roll20 is a
1) Whiteboard
A piece of software capable of displaying and interacting with images. In the case of Roll20, the whiteboard comes into play when you create a campaign. At the top of the screen is a little blue icon that pulls open the pages toolbar. A campaign can have multiple pages each setup with their own thing.
A page has a map layer, a token layer, a gm layer (not visible to the players), and Dynamic Lighting layer (if you have a subscription). The map layer is fixed and anything placed on there can't be be moved except by the GM if he switches to that layer. Everybody sees the map layer. The token layer is used for stuff that the GM and players move regularly mostly character and NPCs miniatures. The GM layer is visible only to the GM and usually used to store stuff like random encounters and hidden characters. Dynamic Lighting is a special layer that is optional and is used to setup a map for use with the Dynamic lighting feature of Roll20.
A page also has fog of war where the entire map will be blanked out except for the areas revealed by the GM. The reveal process works just like drawing squares and shapes on a paint program.
For referee who don't want to use miniatures the pages act like a sheet of paper.
2) A chat and VOIP engine
To the side of the page is a tool bar which has chat. Any thing you type can be seen by the other participants including dice rolls. There are options for hidden whispers and hidden rolls. The setting of the campaign can be enabled for VOIP. It OK most people wind up using Google Hangouts, Teamspeak, or Skype as they have a superior set of features and often superior voice quality.
Note the dice rolling of Roll20 is handled by Roll20 subscribing to a source of random noise generated by quantum fluctuations. Apparently for scientists who need a true random number generator this is a service they can buy. It has obvious applications for roleplaying games.
3) Forum
The Campaign homepage also serves as a discussion forum for that campaign. Used either for updates or for campaigns that are primarily play by post. As I understand it play by post players will jump into the campaign chat and roll the dice there noted which post the roll is for.
4) RPG Utility
To the right of the pages is a toolbar with icons for chat, art (for the GM), Journal, Compendium, JukeBox, Decks and Tables, and Settings.
Art is a search utility for tokens, maps, and images. Roll20 has a webstore with images and tokens for sale. But this will also search the internet for any free resources. You selected it can make it part of a Journal entry or a page.
Journal are notes and character sheets. A note can be text, art, or both. They can be shown to everybody, kept hidden so only the GM can see, or just shown to one player or a limited group. Character Sheets depends on what game is selected. There is a generic version that can be used for any game and a specific version that can be selected in the campaign options.
If you have a subscription to Roll20 and are savvy in designing web pages you also get the option of creating your own character sheet.
Character Sheets are mostly fill in the blank forms however an increasing number feature automation of various rolls. For example in the D&D 5e character sheet clicking the attack button next to a weapon will make an attack roll with all the relevant modifiers applied.
The Compendium displays the SRDs for various RPGs under an open license. Notably Pathfinder, D&D 5e, and I think Fate. It just a hyperlinked document that has the published rules for easy reference.
The Jukebox is a list of free sound and music you can play during the game. For example if you want a wolf howl you can search for it and get a dozen possibilities. You can do this before the game and save your favorite for quick reference.
Decks and Tables allow you to enter random decks or tables. Decks have X choices and when you make a roll you pull the result from the deck (just like a card). It remains out until you reset the deck. Random table works like most random table generators on the internet. Has a list of choices with various odds, makes a random roll, and displays the result.
By combining a Whiteboard, Chat/VOIP, Forum, RPG Utility, Roll20 allows you play tabletop RPG with all the same tools and techniques you use for face to face game. It downside is the same for all social interactions over the internet. The lack of face to face contact makes it impersonal and cold for some. The upside is that it has some tools, like fog of war, that are way easier to use compared a face to face session.
People can and do freely switch between using Roll20 and playing face to face with the same campaign. It that compatible is nothing like a MMORPG like World of Warcraft.
Roll20 is part of a general class of software called Virtual Tabletop. Fantasy Grounds is it's main competitor. Fantasy Ground is stronger in its the RPG Utility feature however it is a standalone program that requires a license from all participants or a pricey Ultimate license paid by the GM. It is available on Steam for those who use that service. It also require minimal knowledge of how your internet router works as you will need to open up a port to allow it function as a server to host your game.
Note that there are other VTTs like VASSAL (open source and free) and Aide-de-camp that are oriented towards board/war games.
There is also Tabletop Simulator on Steam which uses a 3D engine to setup a virtual table with pieces you manipulate. For example if you played Axis and Allies with VASSAL you would be moving icons around. With Tabletop Simulator you would see a table with the board with models of the same pieces as the physical board game. You would move the models around just like you would normally Dice rolling involves rolling models of dice. Oh and you get to throw the table if you lose. Personally it neat for board games. For RPGs I like Roll20/Fantasy Grounds better. It does have a nice Dwarven Forge style setup for RPG so if you are into miniature it may work out better than the image/icon approach of the VTTs.
