AIDA (awareness, interest, desire, action)

A customer passes through four stages before purchasing a product (or service):

Awareness:

They become aware that the product exists.

Interest:

They develop an interest in the product and want to know more about it

Desire:

They now actively want to buy the product and need to know how that can be done.

Action:

They take the necessary steps to buy the product.

Marketing has a role to play in each of these steps. Effective promotion makes them aware of it; good product definition and supporting materials generate and aid interest; well-defined pricing and buying processes help them generate desire; finally everything must be right at the final hurdle to help them take action.