The strings of the oud were traditionally plucked using an eagle's feather, known in the Arab world as a risha (and in Turkey as a mızrap). However, eagle feathers are not readily available nowadays, so modern players have looked to other materials from which to fashion a pick. Many professional players use a risha made from horn (of a cow, for example), which is softened first by being soaked in oil. Plastic rishas (see picture below) are also very popular with beginner and professional alike due to their cheapness and ready availability. If all else fails, you can always cut out your own risha from a plastic bottle!
The risha is held between the fingers as shown in the photos below, with around 1cm protruding. The strings of the oud are struck with the tip of the risha - downstrokes, upstrokes or a mixture of both are used, depending on the player's preference or the desired effect (downstrokes tend to produce more volume and a sharper 'attack'). Rapid alternation between downstrokes and upstrokes is known as tremolo. A good risha technique is one of the main signs of an accomplished oud player, so it is worth spending time on this aspect of your playing.