Is land form caused by deposition or erosion?

Both.
While most of the landmass of Earth is made of igneous and metamorphic rocks, most of the rocks that we can actually see (i.e. those on the surface) are sedimentary. Sedimentary rocks are formed from deposition by forces like rivers, waves and glaciers. Sediment, however, comes from eroded material. The same forces that deposit material also erode material. Most sand in deserts and on beaches is made by silicon oxide, otherwise known as quartz. Quartz is an extremely common mineral in igneous (volcanic) rocks. The sand comes from eroded igneous rocks, mostly granite. Quartz happens to be more resistant to erosion than other minerals in granite, like feldspar and mica which erodes to clay-like substances.

If you're talking about the landforms themselves, then it's still both. Landforms are usually categorised as being erosional or depositional in origin.