I have a confession: my group don't meet in person for our sessions of D&D, we play online. I realise that this may be blasphemy to some people, but with the range of modern technology available to us, I honestly feel that it's a perfectly valid way to play.
Admittedly, certain aspects of the game are hard to replicate in a virtual environment; the phenomenon of players getting attached to “lucky” dice doesn't really happen with random number generators. Nevertheless, for my group, the convenience of playing online was worth missing out on a few RPG traditions.
D&D is quite a social experience and of course there are many ways to communicate online. Some games are played on forums in a play-by-post (PbP) style or even via email, naturally these can be quite slow paced but may be ideal for groups who can't all play at the same time. For faster paced games, like you'd normally expect when played at a table, real-time chat of some variety is needed. Our group uses voice chat, but web-cams or text chat would probably work too.
For D&D 4th Edition, at least, much of the game revolves around grid-based maps. Sharing images online is easy enough, but for a more interactive solution there are programs known as Virtual Gaming Tables or Virtual Tabletops.
My group uses a program called MapTool, which has many features, but essentially allows DMs to create maps while players can connect and interact with them in real-time. MapTool is free, open-source and written in Java. It should run on Windows, Linux, Mac OS X and just about any other system with a Java Runtime Environment.
Creating maps is as simple as dragging and dropping tiles from the resource library, rotating and resizing as needed. The default library of artwork is small but several other packs are available to download from the “Add Resource to Library” option in the File menu. MapTool also seems to support most of the common image file formats, so you can use just about any image you can find.
Tokens are used to represent player characters, NPCs and monsters. Besides a picture, they can be used to store information, from simple notes to an entire character sheet. Each token has ownership settings, so players can be restricted to controlling only their own tokens. MapTool also includes Vision Blocking and Fog of War, which limit players to seeing only what their character's token should be able to see.
Despite the name, MapTool is more than a mapping program. There are too many features to mention in detail, but of course there's also text chat, initiative tracking, dice rolling, whiteboard style drawing tools, area of effect templates and so on.
MapTool tries to be “game system neutral” by not having much built-in support for specific games, so that it can be used for lots of different games. However, it's very configurable and there's an active community that create and release macros and frameworks that can add all kinds of features.
My group have been playing online with MapTool since we began playing D&D, I can't recommend it enough. We've added a few macros and frameworks to our arsenal as the weeks went by, and now our games run really smoothly. Honestly, if your group is ever unable to gather in person, consider moving the session online instead.