There are two main "families" of silicone - tin-based (also known as "condensation cure") and platinum-based (or "addition cure") silicone. In each "family" you can find a wide variety of silicones, with many different grades of hardness, softness, curing and working times, elasticity etc. But in general, all tin-based silicones share a certain set of properties, and all platinum-based share certain properties.
Tin-Based Silicones : are generally less expensive than platinum-based silicones are not prone to inhibition (ie have environmental factors prevent curing) are usually used for making molds (although they can be prone to shrinkage) are NOT suitable for use against the skin even in a cured state cured pieces have a short "library life" - meaning that after a few years the piece loses its elasticity and becomes brittle. Platinum-Based Silicones: cured pieces do not become brittle or break-down over time making them suitable for long-term art pieces and molds are prone to inhibition (ie a variety of factors can prevent the silicone from curing) skin-safe once cured (certain types can be applied and allowed to cure directly on the skin) show very little shrinkage when used to make molds once cured, platinum pieces have greater temperature and chemical durability than tin-based pieces