In one sentence, describe what the following article is about:

Right-click Modloader, and then select Open with → WinRar (or another archiving utility). These are your .class files. Keep this file window open. In Windows XP: Click on the Start button, then click on Run. Type "%appdata%" and click on "Roaming." Minecraft should be the first folder. Open it. In Windows Vista/7: Click on the Start(Windows) button and type "%appdata%" in the search bar and again click on "Roaming." In Linux Ubuntu (should be the same on other Linux distributions): Open up your Home folder. Look for the Minecraft folder. Note: if you cannot see this folder, enable "Show Hidden Files". When you find it, open Minecraft.  jar file with WinRar or a similar archiving utility. Right-click minecraft → Open with → WinRar. This step is very important. If you don't adequately delete the META-INF file, your screen will go black when you try to play Minecraft. class files, including folders, from the Modloader folder into the minecraft.jar window. jar window and run Minecraft.exe. After half a minute, exit out of Minecraft and check your minecraft.jar window again. If you see a new folder entitled "Mods," the installation of Modloader worked. Download mods and continue to enjoy Minecraft.

Summary:
Unzip Modloader with your archiving utility. Open up the Minecraft folder using one of the following three steps, depending on your operating system. Open the Bin folder by double-clicking it. Open the minecraft. Delete the META-INF file. Drag and drop the . Close the . Log in and start playing for a short duration.