Market, planned and mixed economies
Three main sets of decisions need to be made by the economic system - what to produce, how to produce, and how to share out the product of the economy. A market economy is one in which answers to the three questions above are determined by buyers and sellers interacting with each other without government interference. By contrast, a planned economy is one in which a central planning agency such as the government makes the major economic decisions. A mixed economy includes elements of both the market and the planned economies.
Those (and there are few of these left today) that favour the centrally planned economy argue that the government (central planners) are best placed to meet the needs of all the people of a particular society. Those in favour of the free market argue that central planning wastes resources and that the market makes sure that consumers get what they want produced, while competitive firms supply it at a profit. The reality is that most societies operate some form of mixed economy.
In the UK we have a mixed economy. Most decisions are made by the market - e.g. when you buy goods in supermarket you vote with your money for the goods that you want to buy. However, some decisions are made by the government e.g. those relating to road building, school and hospital construction, the supply of medicines in hospitals etc.
In the UK, the emphasis is on letting the market make most decisions because of its high level of efficiency in responding to customer preferences. However, some decisions must be made by the government on behalf of society e.g. decisions about military spending, and public education.