A pixel is a single point on a display device, used in raster graphics, such as PNG or JPEG files, and often to control the dimension and position of elements on the screen.
Pixels are also used in UI design, when placing elements precisely, though relative units can also be used.
Pixels are also used when sending mouse clicks, positioning, and resizing windows in AutoIt.
Many developers prefer using a relative unit, such as Ems or percentages (%), even though pixels should no longer present much of a problem, since most browsers allow to zoom in on the entire website, and you can also use media queries to adjust the layout for different window sizes.
Using media queries to control page layout in different window sizes effectively eliminates the need to create a separate mobile version of your website.