Indian tradition holds that a town or village without a temple is uninhabitable. The Mandir is not primarily considered a place for communal worship but the home of God, or the particular Deity. Temple activities thus revolve around the sacred image(s) installed upon the altar.
An appointed priest, or team of priests, normally perform the puja. One of the main functions of the temple is to create an atmosphere surcharged with spirituality and hence temples are often built on holy sites. During quieter periods the temple provides opportunity for peaceful reflection. At other times, it is a bustle of noise and activity.
Temples vary considerably in size, beginning with tiny outdoor shrines and humble village mandirs. The larger temples are elaborate and often the centre of an ashram (place of spiritual culture) with a large number of brahmana priests living within or nearby. The temple of Balaji in Tirupati ( South India) is reckoned to be the largest, with a total staff of over 6,000.
There are a number of architectural styles, but the chief ones are North Indian (Nagara) and the South Indian (Dravida). Details for temple construction are laid out in certain scriptures such as the Vastu-shastra and the Shilpa-shastra (see Shruti: The Four Vedas). To help build a temple (e.g. through offering financial support) is still considered an act of piety.
In the UK, the first temples tended to be converted public buildings, such as school halls and churches. They often have an orange flag flying from the roof. Now there are an increasing number of purpose-built mandirs, in both modern and traditional styles (or a combination of both.) Temples in Britain, unlike those in India, often double up as community centres where Hindus can meet and organise social, cultural, and charitable events. Throughout the UK, there are now about 150 temples featuring regular worship.
Temples vary considerably, but the diagram here show some of the important features of traditional buildings in North India. Buildings in the South or outside of India are often quite different.