Amines can be classified as primary, secondary or tertiary by the number of alkyl or aryl groups attached to the nitrogen atom.

• Primary: one alkyl or aryl group bonded to N.
• Secondary: two alkyl or aryl groups bonded to N.
• Tertiary: three alkyl or aryl groups bonded to N.

The nitrogen atom of an amine has a trigonal pyramidal shape with bond angles of ≈107°.

In aliphatic amines, the nitrogen atom is bonded to at least one straight or branched carbon chain i.e. an alkyl group.

In aromatic amines, the nitrogen atom is bonded directly to a benzene ring.

Amines behave as Brønsted-Lowry bases because the lone pair of electrons on the nitrogen atom can accept a proton, forming a dative covalent bond.

For example, ethylamine behaves as a Brønsted-Lowry base:

C₂H₅NH₂ + H⁺ ➔ C₂H₅NH₃⁺

Amines neutralise acids to form salts. 

For example, ethylamine reacts with hydrochloric acid to form ethylammonium choride:

C₂H₅NH₂ + HCl ➔ C₂H₅NH₃Cl

The availability of the lone pair of electrons on their N atom to bond with an H⁺ ion.

The more readily available the N lone pair, the stronger the base.

Base strength increases in the order: primary aromatic amines < ammonia < primary aliphatic amines.

The positive inductive effect of the alkyl group increases the electron density on the nitrogen atom, making nitrogen's lone pair of electrons more readily available to bond with an H⁺ ion. 

The lone pair of electrons on the nitrogen atom overlaps with the delocalised system of the benzene ring, making the nitrogen lone pair less available to donate.

Quaternary ammonium salts are complexes formed by quaternary ammonium ions and negative ions.

(CH₃)₄NBr is an example of a quarternary ammonium salt.

Quaternary ammonium salts are used as cationic surfactants in fabric cleaners and hair products because they can bind to non-polar substances like grease and dissolve in water.

The hydrocarbon tail binds to non-polar substances and the cationic head dissolves in water:

The positively charged ammonium ion can bind to negatively charged surfaces like hair and fibre. This eliminates the build up of static electricity and keeps the surface of the fabric conditioner smooth.