Squalene is an oily liquid that originally comes from shark liver but luckily it can also be found in a couple of plant oils. Olive (0.6%), peanut (0.1%) and pumpkin (0.35%) oils contain it, though not in huge amounts. 

What contains more of it, is the sebum (the oily stuff) that our skin produces. About 13% of human sebum is squalene, which means that it’s an important skin-identical ingredient and NMF (natural moisturizing factor)

Chemically speaking, it is an unsaturated (has double bonds) hydrocarbon (contains only carbon and hydrogen) molecule, that can undergo oxidation. On the pro side, this means that squalene can act as an antioxidant (while its no-double-bond version sister, squalane cannot) that might be useful for dry skin types, but on the con side, oxidized squalene (called squalene peroxide) is thought to be a strong acne-trigger, that makes squalene less ideal for acne-prone skin.

Having double bonds, and being prone to oxidation also means that squalene is a less stable molecule with shorter shelf life, so its more stable and awesome sister, squalane shows up more often on ingredient lists. Read about squalane here >>