A prism and a pyramid both have three dimensions: they both have a cross section and a height. Mathematically speaking, a cross section is a plane formed by cutting across a solid. When you cut a prism and a pyramid across, you have a plane. Because both solids are three-dimensional, both can hold solid, liquid and gas, and you can calculate their volume from mathematical formulas.
The bases, or ends, of a prism and a pyramid are polygons. A polygon is any two dimensional shape with straight sides -- rectangles and triangles, for instance. For their bases, both a prism and a pyramid can have a polygon of any dimensions. If the dimensions of their bases are identical, a prism and a pyramid will have identical base areas but their volumes need not be identical, too. Their volumes will depend on their cross-sectional areas and heights.
Unlike a prism, whose faces are always parallelograms, a pyramid always has triangular faces. A parallelogram is a two-dimensional shape with four lines and with opposite lines that are parallel to each other. The faces of a pyramid are all slanted, meeting at its top or apex, whereas the faces of a prism are all vertical and never meet because they support two parallel polygons (the ends) at the edges of the polygons. Parallel shapes never meet, so faces supporting parallel shapes at the edges of the shapes never meet.
Though there are different types of prisms and pyramids, every prism is always different in shape from every pyramid. Unlike prisms, which always have identical polygons as their ends, pyramids taper toward their upper ends. You can, therefore, stand a prism on either of its ends whereas you can stand a pyramid on only one of its ends. With this special dissimilarity, you should have no difficulty distinguishing a prism from a pyramid.