Jack has beaten me to it. Geothermal energy is particularly important in Iceland where almost all of their energy – and Icelanders are among the heaviest users of energy per person in the world (there just isn’t very many of them). In the UK, if you did down about 20 metres, there is sufficient heat difference from the surface that, using a heat pump (effectively the reverse of a refrigerator), it is possible to generate useful amounts of heating.
Some good answers here – there are two types of geothermal energy which can be exploited in the UK. As Laurence suggests – if you dig down deep (20m or so) you reach a point of almost constant temperature regardless of the season or weather on the surface. The temperature difference between the deep pipe and the surface can be used to reduce the amount of energy needed to heat buildings in winter, or cool buildings in summer. The other type of geothermal energy is to coil long lengths of piping across a big area (e.g. under a car park or large garden). The effect is similar but not such great depths are needed, so this is often a cheaper option..