Mandalas are patterns that expand from a single central point. The patterns extend outwards from the centre in rays like a star or a flower. These designs are most often highly geometric designs with a very precise symmetrical layout. A Mandala design can also be used as an ornate background for a non-symmetrical central design like an animal or characters, see the example below
Large flat areas of the body like the back are the favoured location for these types of tattoos because this shows off their impressive details and symmetry. However good tattoo artists can apply these designs to smaller areas where the bodies natural curves and changes in viewing perspective can give the tattoo added interest. The image below is an excellent example of this. Large Mandalas on the shoulder also work really well.
Of course, Mandalas can form part of a larger design scheme. In the tattoo below you can see that the sleeve radiates from a Mandala on the shoulder. Because these designs need to be symmetrical you will probably find that your tattoo artist uses computer assisted design or stencils to layout the main framework as small errors in the layout will look terrible.
Mandala tattoos tend to be based on very intricate dense patterns resembling lace. These design features can be used in tattoos with other shapes like the one below however they are not true Mandala tattoos because they lack the radiating spoke structure of the Mandala.