= Solution
A <screw axis> combines rotation about a line with translation parallel to that line. An $n_p$ screw rotates through $2\pi/n$ and advances by $p/n$ of the lattice repeat along the axis; a $2_1$ screw advances by half a repeat after a half-turn. The combined operation, rather than either component separately, must map every atomic species onto equivalent sites.
A <glide plane> combines reflection in a plane with a fractional translation parallel to it. Reflecting alone need not preserve the crystal. A <crystallographic point symmetry> is an isometry with a fixed point, such as a rotation, reflection, inversion, or <rotoinversion>. Point operations have no irreducible translational advance: a screw or a glide with nonzero fractional advance cannot be described merely as a rotation or mirror at a different origin. A crystal's <point group> records the rotational parts of its space-group operations, whereas the full <space group> retains their translations.
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