Normal gameplay is as follows: Notes move up or down towards the target on a usually static background, and the player hits the notes when they reach the target. Simple.
Effect simfiles change this up, which can result in interesting gameplay results.
There are multiple 'catagories' of effect files:
"Gimmick" charts
"BGA" charts
"Foreground" charts
"Modifier" charts (or "modcharts"/"modfiles")
These are not mutually exclusive categories. One simfile could be a combination of "Foreground" and "Modifier", while another could be a combination of "Gimmick" and "Foreground".