It is a human trait comprising of a complex phenomenon that is composed of some elemental traits in combination, things like envy, possessiveness, and extraversion.
It is an inordinate desire to own things. The trait is exploited by marketers by holding all kind of Annual Clearance Sales, Holiday shopping Seasons or Auspicious buying days etc.
As a value, materialism is opposed to the spiritual; in a sense, it elevates the acquisition and possession of objects to a primary value, akin to a religion.
Some western philosophers have promoted it as a way of life. Materialism results in increased consumption which leads to higher Economic growth. America is a prime example. Whether this consumption leads to happiness is a moot point.
World over it is encouraged. Luxury holidaying is an example to extreme materialism. Expensive weddings are another example. It may horrify people who profess austerity but if everyone in the world became austere, a lot of people in the world will be jobless.
Consumerism is about protecting the rights of the buyers. Unscrupulous businessmen tend to exploit the consumers. Real Estate industry in India is a prime example. Consumerism is to put systems, laws etc. in place so that customers are not cheated.
Some companies are so focused on cheating that they have elevated their ways to the level of Art form. They will use tricks like complex buyer agreements, lawyers skilled at delaying proceedings in the Court, billing the same item multiple times by different names, marketing the same item under different brands through their different companies to avoid action by anti-monopoly laws.
The activism of consumers in doggedly pursuing remedies and pressurizing Governments to bring in new laws is part of Consumerism.