There are many types of meditative stabilisation, but let us explain calm abiding (samatha) here. The nature of calm abiding is the one-pointed abiding on any object without distraction of a mind conjoined with a bliss of physical and mental pliancy. If it is supplemented with taking refuge, it is a Buddhist practice; and if it is supplemented with an aspiration to highest enlightenment for the sake of all sentient beings, it is a Mahayana practice. Its merits are that, if one has achieved calm abiding, one's mind and body are pervaded by joy and bliss; one can--through the power of its mental and physical pliancy--set the mind on any virtuous object one chooses; and many special qualities such as clairvoyance and emanations are attained.
--from The Buddhism of Tibet by the Dalai Lama, translated and edited by Jeffrey Hopkins, published by Snow Lion Publications