What is Screw axis, glide plane, space group?

A screw axis $n_p$ combines a rotation by $2\pi/n$ with a translation of $p/n$ of the lattice repeat along the axis ($2_1$: half a turn and half a cell). A glide plane combines a reflection with a translation of half a lattice vector parallel to the plane ($a$, $b$, $c$ glides). The group of all the symmetry operations of a crystal, translations included, is its space group: there are 230. Its smallest part from which the whole crystal is generated by the operations is the asymmetric unit. A set of points left on themselves by a subgroup of the operations is a Wyckoff position: general positions have no symmetry, special positions lie on symmetry elements.