Figure 1. Space filling model of methane. The white is hydrogen
and the black is carbon
and methanol (note the oxygen
, in red, is the only difference between the molecules).
Hydrocarbon derivatives are very similar to hydrocarbons, but an atom in the original hydrocarbon gets something (usually another atom) else substituted in.
For example methane becomes the hydrocarbon derivative methanol when one of the hydrogen gets replaced with an oxygen-hydrogen (OH) group, see figure 1. Note that methane is a gas and methanol is a liquid at room temperature.
Hydrocarbon derivatives can also be used as fuels. For example ethanol, which is often made from biomass (and often referred to as bioethanol) has an OH group substituted for one of the hydrogen in ethane.
To learn more about hydrocarbon derivatives please see hyperphysics or UC Davis's chem wiki.