An EGA is a video adapter for IBM-type personal computers. This means it has the electronic circuits your computer needs to display images on the screen. EGA stands for enhanced graphics adapter, but don't be fooled by the name: EGAS came out in about 1985, and what was "enhanced" then (compared to a CGA) is obsolete now. Yes, they still work, and if you get a computer that has one, don't throw the EGA board away.
Most new graphics and Windows programs will function with an EGA. But don't buy one new-get a VGA instead, or better yet, a Super VGA. In graphics mode, EGAS can display a maximum of 640 by 350 pixels (dots of colored light) on the screen. That resolution is barely acceptable, and it produces squashed looking images (squares look like rectangles). EGAS can display text too, but EGA text is a little too fuzzy to read for long periods of time. Though a breakthrough at the time, by today's standards EGA color is limited: 16 colors from a palette of 64 colors can be displayed on the screen at one time.
You'll hear people talking about their "EGA adapter," but that's probably redundant, since the A in EGA stands for adapter. On the other hand, an "EGA screen" or monitor is a monitor designed to work with an EGA