A launcher for Android handles the way you interact with your phone. It decides where you can put your app icons, and how the touch menus interact with the user. Basically, it's what decides that your App Drawer button should look like a group of dots, or that your browser icon should be a round globe.
If you've had an Android phone for a long time you might want to try different launcher applications to switch things up. Alternatively, if you're converting from a Windows Phone 7, one app can make the transition quite a bit easier.
Here's how to get started:
Download and install Launcher 7 from the Android Market (it's free, or you can choose the donate version). Once the app is on your phone, all you need to do is hit the Home button to see an option to use it, like this:
Pick Launcher 7 instead of any other choices you may have, but don't set it as the default until you're happy with the way it works.
Now it's time to customize the look and layout of your new tiled launcher. Tiles can be switched around by pressing and holding on one and dragging it to another open spot. This is why the launcher starts out with one empty square, to make the adjustments easy the first time. If you decide that you don't need one of the icons, they can be removed by pressing and holding the tile and then tapping the thumbtack with a line through it in the upper right-hand corner of the tile.
Choosing the small icon in the upper left-hand corner opens a full set of editing options for the tile. You can choose a custom name, icon, color, and even the option to show unread counts for messages in apps like Gmail or SMS.
If you're looking for your other apps that aren't on the tiles, simply tap the arrow at the top-right of the black bar. Tap on an app to open it, or press and hold to get the option to add it to the main tiles.
One last thing: you can scroll down to add/see more tiles, or you can adjust the launcher to only contain enough tiles for one screen's worth.