A Hydrocarbon is any compound with molecules each consisting of just Hydrogen (H) and Carbon (C) with no other element. Hydrocarbons are found in crude oil and are the primary constituent of refined fuels. On Earth, they are thought to have been generally formed from material of biological origin, but they are present in the output of deep ocean vents and they are also present on other bodies in the solar system which are presumed to be of non-biological origin. Thus, to an extent a hydrocarbon is a Biosignature, but what's considered more reliable is ratios of specific hydrocarbons with others and other material.
Carbon atoms tend to Bond to four other atoms, each of which can be carbon or hydrogen (or other element, in which case it is no longer a hydrocarbon), and hydrocarbons can be so-constructed in various ways, including long chains, and branches. Some classifications:
The simplest Alkane is Methane (CH4) with one carbon and four hydrogen atoms. Next is Ethane, with two carbon atoms, each bonded with three hydrogen atoms.
Polycyclic Aromatic Hydrocarbon (PAH)