An infinite variety of compounds can be assembled from only carbon and hydrogen atoms. So-called hydrocarbons may be the simplest organic compounds, but they have mighty economic importance because they include stuff like petroleum and natural gas.
Propane, butane, and isobutane are all hydrocarbons with only single covalent bonds between carbon atoms. These hydrocarbons that lack double bonds, triple bonds, or ring structures make up the class called alkanes. The six simplest alkanes are methane, ethane, propane, butane, pentane, hexane, and windowpane. (Oh, wait... one of those doesn't seem right . . .)
An alkene is a hydrocarbon with at least one double bond between carbons. The simplest alkene is ethylene, C2H4. As is the case with the alkanes, each carbon has precisely four bonds to fill its valence orbitals with eight electrons.
A hydrocarbon with a triple bond between carbons is an alkyne, and the simplest compound in this class is acetylene, C2H2. Once again, each carbon has exactly four bonds. Of course, the triple bond between carbons allows each carbon to bond to only one more atom. In acetylee, the single bond is to hydrogen, but in other alkynes, the single bond is to another carbon.