GIS Files Extensions - What are gis files and how do I open a gis file?

What are gis files and how do I open a gis file?

File Type Category File Format
.apl GIS Files ArcPad Layer File
.avl GIS Files ArcView Legend File
.dlg GIS Files Digital Line Graph
.est GIS Files Streets & Trips Map File
.mwx GIS Files MapGuide Author Map Window XML File
.saf GIS Files Street Atlas USA Map File
.stt GIS Files Streets & Trips Map Template
.tab GIS Files MapInfo TAB File