A Mala is a closed string of 27, 54 or 108 beads and one (often) larger bead, called the Guru bead that hangs perpendicular to the rest of the Mala. The tassle represents the awakened Sahasrara (crown chakra) or 1,000 petaled lotus as a symbol of enlightenment obtainable through the practice.
Mala beads can be used to help keep count of your mantra, or of your breath through a Japa meditation cycle. In a full cycle of Japa you begin with the bead next to the guru bead , and slowly feed the beads towards you; pausing at each to recite your mantra or to take a breath. Working through a full cycle of 108 repetitions can take anywhere from 5 minutes and is an immensely powerful practice to affirm your intention, however, at times where a full cycle is not possible the practice can be abbreviated by simply counting back from the guru bead for say 7, or 27 beads and then beginning your practice.
Different traditions observe different respects when counting the mala. In Hindu practices the mala is always held in the right hand, with the beads resting over the middle finger of the hand (the index finger is never used and is sometimes called the admonishing finger!) with the thumb used to count. When the guru bead is reached it is neither counted, nor crossed, rather the bead is intentionally bowed to, the mala is turned around and the count begins back in the opposite direction. Always towards the guru bead as the representation of the supreme conscious.
Tibetan traditions do not follow these same rules and instead count the beads on either hand, and with any digit. However it is interesting to consider that different fingers contain different meridians or acupressure points that are stimulated by the passing of the beads adding an extra dimension to your meditation.
The index finger bestows wisdom, knowledge and prosperity,
The middle finger, encourages patience and trust,
The ring finger stimulates overall good health and fresh energy, strengthening the entire nervous system, and
The little finger creates more intelligent communication.
Of course a mala isn't necessary for any of these practices. If you need to count there are many techniques that count through the fingers in varying amounts (have a look HERE).