mantra mean "powerful word". mantras are the ones that have when chanted produce great effects. These are chanted repeatedly and that is called japa. japa is a key part of Hindu prayer.
mantras are very rich in their meaning. While doing japa one can meditate on the mantra - its meaning. As the mind dwell more and more into that, the mantra conditions the mind and takes up to the higher states and forms the path to the great liberation - eternal bliss !
What makes mantras so special as compared to the normal words ? mantras are not human composed. One may wonder how can that be possible. Especially given that there are sages associated with the mantras ! The point to be noted is that these sages are not composers of these mantras, as we normaly compose the sentences; they are not the inventors, but they are the discoverers of the mantra. They get to know the mantras in a state in which these words do not emanate from their thoughts, but they are just passive audience to it. Those who go deep in meditation and realize God may be able to get a feel of this situation.
To be such a discoverer, even though they are just passive hearers, needs great amount of qualification. Only the perfect one can unchangedly reproduce the mantra heard. The only one that is absolutely perfect is God. All other discoverers reproduce that mantra only as pure as their closeness to perfection.
veda samhitAs are full of mantras and hence have been preserved for ages in their pure form by utilizing the various techniques like patha, krama, jaTa, gaNa pATas, that ensure that the chanter clearly gets the correct letters and even the correct level of sound for each letter (svara). The chanters are advised to chant the mantras only after getting the right pronunciation of it, so that the mantras are presered against deterioration with time. There would be gurus who initiate the disciple in a mantra. guru ensures that the disciple got the mantra right, so that the person can chant independently as well as initiate others in that mantra. Ensuring this preservation vedas were passed only through the tradition of guru and disciples and was never written down till very recent past. (It is really amazing to note that without being written down the vedas have been preserved in pure form across the land by these techniques. Though the texts arem across the land by these techniques. Though the texts are freely available now for anybody to read, it would be important to ensure that these mantras are properly learnt and then chanted. This way the treasure that as been preserved so carefully over multiple milleniums do not deteriorate due to indifference.)
It is to be noted that many of the hymns of thirumuRai(1) are known to have great powers of mantras that are practiced even today.
While there are plenty of mantras available, there are a few that are chanted with high esteem by the shaivas. Definitely those are highly powerful ones that can lead the chanter on the great path to mukti (liberation). praNava, paNJchAkashra, gAyatri to name a few. For shaivites the Holy Five Syllables (paNJchAkshara) with or without combined with the praNava is the ultimate mantra.
Pancha brahma mantras
As the mantra of four vedas, staying in the mind of celestials - ruling them, the mantra for the rite growing perfect vedins for the twilight prayers, is the Holy Five Syllables (namaH shivAya). - thirumuRai