Emulators are programs that allow one type of game to be played on another system. Such as a PS2, or PSP native game being able to be played on a PC. While not just for video games, they can be used for other purposes. Some parts of emulating setup is prohibited under law such as obtaining copies of BIOS (Basic Input Output System) unless these were purchased legally. Emulating is regularly a complex thing to like like modding a console game system. Also emulators are a benefit when running the game in it's original form.
The graphics are the same on the PC. However some problems do arise (like with GTA games). One of these problems is with the graphics. Such as the radar, and HUD not being rendered, and black boxes can be seen around them. Also the game can contain distorted graphics as opposed to the PS2 on a television monitor. This cannot however be fixed with in-game anti-aliasing as it doesn't exist.