"I See You, Mara!"

So said Shakyamuni Buddha on his night of Enlightenment when Mara, the Tempter and Deceiver tried to prevent his enlightenment. But the Buddha saw Mara for what he was, the Buddha was not tempted or deceived and proceeded on to Full Liberation. Even though Mara was seen and defeated at that time, he occasionally reappeared during Shakyamuni's subsequent 45 years of teaching the Dharma. The point is that if Mara was lurking around the Buddha even AFTER his enlightenment, then we should certainly keep ours eyes open also.
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Mara deceives people and tempts them away from the path to Buddhahood. In doing his work, Mara takes many forms, but the following four are the principle ones:
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1. The Mara of Death. The fear of dying, or being directly faced with death, can cause people to veer off the path to Buddhahood.
2. The Mara of the Five Skandhas. Ordinary untaught people cling to the skandhas and are therefore unable to experience the reality that the SELF is unreal. Mara would have them believe that the self (the atman) is real, and thus prevent them from enlightenment.
3. The Mara of Worldly Cares. By keeping people entangled in the worries (such as money) and busy-ness and klesas of THIS WORLD, Mara robs them of the time it takes to learn, enter, and explore the OTHER WORLD, which is where Buddhahood is experienced.
4. The Mara of Pleasures in the Heaven Realms. On the night after six years of striving, when Shakyamuni sat in lotus posture to make the final push to realizing Nirvana, Mara appeared and said, "Relax. No need to strive. You are already enlightened. You are known throughout the world as 'The Great Ascetic Gotama.' Striving is pointless, and if you return to the WORLD now, you will become a Great Emporer, you will have happiness, wealth, and fame, and the most beautiful and talented women will be your loyal companions." But the Buddha clearly saw Mara, and was not moved from the still concentration of mind.