A seven sided figure with equal angles and equal sides is known
as a heptagon. It looks similar to a pentagon but has more sides
and therefore larger angles.

There are many possibilities, and here are a couple: A dodecagonal prism (dodecagon = 12 sided plane figure); A heptagonal pyramid (heptagon = 7 sided plane figure).

A seven sided figure would be a heptagon. A Reuleaux heptagon, is a seven sided figure, but having constant diameter. [As seen in some British coins]. But a regular heptagon would have seven equal sides that were each straight.

Most polygons are based on Greek roots. The shape is called a heptagon because the Greek prefix "hepta" stands for seven. The corresponding Latin prefix is "septa". So septagon (with the Greek suffix "-gon" meaning angle) would be a name which based on a mix of Greek and Latin roots.

There's no difference because both words mean a 7 sided shape. However, "septagon" is an incorrect usage based on the supposition that the names of polygons are based on Latin, when in fact they are based on Greek. The correct name in English for a 7-sided, 2-dimensional figure is "heptagon."