Cholesterol  Oestradiol  Cholic Acid
All steroids have the same general tetracyclic ring structure, consisting of three 6-membered rings (ABC) and one 5-membered ring (D) fused together. The numbering system of the nucleus and sidechain remains unchanged even when bridgehead methyl groups are removed.
3D Representation

Naturally-occurring steroids have trans BC and - except for the cardiac glycosides - CD ring junctions, where cis/trans describes the relationship of the two acyclic bridgehead substituents. AB Junctions (represented here in the Decalin models) can be found either cis - as in Digitoxigenin or the bile acid Deoxycholic Acid - or more commonly as trans.
Trivial names have been retained for the different families of steroid nucleus - cholestane, oestrane, etc. Substituents are designated as beta (b) if they are above the plane of the ring system (i.e. coming towards the viewer) or alpha (a) if they are below. Stereochemistry at ring junctions is standardized on all-trans and always implied as such when not explicitly defined.