Alkanes and alkenes are two fundamental categories of hydrocarbon molecules (being composed primarily of a carbon chain with non-chain bonds filled by hydrogen). The difference between alkanes and alkenes is that alkanes contain only single bonds between their carbon atoms, whereas alkenes have at least one double bond. Note that the minimum number of double bonds in order to qualify as an alkene is just one; the molecule can be composed of almost entirely single-bonded carbons, but if just one pair is double-bonded, the molecule is an alkene.
Besides these compositional differences, alkanes and alkenes can be recognized by the different molecular geometries that their bonding relationships generate (the double bond forcing either a trigonal planar arrangement or a linear one depending on what's on the other end of the bond). They also have different naming suffixes; all alkanes end with "-ane," and alkenes end with "-ene."
Alkane carbons will be sp3 hybridized, because they bond to four other atoms and need four equal orbitals to do so. Double-bonded alkene carbons will be sp2 hybridized because they only have three bonded atoms.
Pi bonds are formed only in the alkene because the un-hybridized P orbital is projecting out of the plane of the other bonds, reducing its ability to overlap. The presence of pi bonds are a requirement for electrophilic substitution, so this will only occur in an alkene.
Both molecules can participate in combustion reactions, but alkenes will provide less energy and potentially more waste products due to the additional energy necessary to break the double bond.
Homolytic fission/substitution would necessarily break the alkane apart into two smaller molecules, whereas if this occurred to the double bond in the alkene the molecule would remain intact, but would become an alkane.