This is an excellent overview of the mechanical aspects of playing D&D 4e, something worth keeping to hand for new players joining your group or as a ready reference as you learn to play.
It dives straight in, after a little bit of waffle about D&D having been the original role-playing game, to describing the core mechanic: state what you want to accomplish and roll a D20, applying any appropriate modifiers as shown on your character sheet, and if you exceed a target number set by the DM, you have succeeded in whatever it was you are trying to do.
After describing in a little more detail how an attack in combat is resolved, the concept of any challege that might need resolution being an 'encounter' is explained. Outside of encounters, characters explore and engage in social interactions, during them things get a bit more mechanical and the dice come out. In noncombat encounters there still is scope for more than die rolling with skill and power use and roleplaying contributing to task resolution.
Powers and levels are briefly explained, and then it's back to combat with a more detailed exploration of what goes on: establish where everyone is, roll initiative, act (and resolve actions by die rolls) in turn, repeat as necessary. Surprise is dealt with, then a closer look at what you get to do when it is your turn, and then what you can do when it is someone else's turn - a bit more than sit and watch, or go make coffee, because you might get an attack of opportunity or other chance to do something.
The discussion on combat - which after all is the hardest part for a newcomer to grasp because this is when it gets rules-intense - continues with criticals, attack types, conditions and more being defined and explained. Then other areas are covered including even some brief descriptions of skills and how you use them.
The whole rounds off with some example characters so you can try these rules out without further ado! It is recommended that you first play with someone who has read the full rules and can guide you through the first few encounters, and you will need the Player's Handbook to be able to generate a character of your own, but this provides an excellent introduction and enough to let you know if you're going to enjoy the game...