What is halogenation?

Halogenation is the replacement of a hydrogen atom by a halogen atom in a molecule.

Halogens is the group name that is given to fluorine, chlorine, bromine and iodine. Since these elements have very similar behaviour, they are often treated as a group. The term 'halogenation' can thus refer to replacing any number of hydrogen atom with each and any of the members of the group.

The product resulting from halogenation will have quite distinct properties from the start compound.

Example:
Methane #CH_4# is a gas, that burns easily
If you chlorinate (=halogenate with chlorine) you get different substances like #CH_3Cl# or #CH_2Cl_2# or #CHCl_3# or #C Cl_4#
named mono-, di-, tri- and tetra-chloro-methane (the tri-version also known als chloroform)

The completely chlorinated compound #C Cl_4#, often called 'tetra' is a strong de-greasing cleaning fluid (with a bad reputation for doing harm to liver and kidneys).

There are several processes to halogenate carbon compounds.

One other group of products are the result of mixed halogenation, i.e. different halogens are used, for instance chlorine and fluorine. This leads to chloro-fluoro-carbons (CFC's), of which #C Cl_3F# is just one example. They affect the ozon-layer, reason why their use is highly restricted.

There are two types of halogenation.

Halogen Addition

An example is the addition of bromine to ethene.

Halogen substitution

Halogens react with alkanes under the influence of heat or light to form alkyl halides.

The halogen atom replaces a hydrogen atom in the alkane, so this is a substitution reaction.

Aromatic compounds undergo halogen substitution reactions in the presence of Lewis acids.

Here's a video on the halogenation of alkanes.