Slopes are developed by varying geomorphological factors.
Basically anything that's inclined is a slope; mountains, hill, valley sides etc
Some of the things causing slope development are
-Tectonic movements producing inclined folds & faults
-Debris accumulation at bottom of scarp. The more spread out the deris the gentler the slope
Now they are factors which affect their development such as vegetation, rock type etc.
There are also theories on how the evolve from steep to flat