What is Torsion angle and conformations?

For four atoms A–B–C–D, the torsion angle $\theta$ about B–C is the angle between the planes ABC and BCD, read on a Newman projection along B–C. A conformation is an eclipsed conformation when the bonds of the front and rear carbons are aligned in projection, a staggered conformation when they alternate. In butane, viewed along C2–C3, the staggered conformation with the two methyl groups at $180^\circ$ is the anti conformation, those at $\pm 60^\circ$ are gauche conformations. In cyclohexane, the strain-free puckered ring is the chair; each carbon carries one axial bond, parallel to the ring’s axis, and one equatorial bond, roughly in the ring’s mean plane. The ring flip converts one chair into the other, every axial bond becoming equatorial and the reverse.