The Vedic Mantras are different from the traditional Slokas in that the Slokas are chanted straight through... there is no need for any intonation, Swaras or whatever; even if you have one, it carries the same import. Not so with the Vedic Mantras. They are set to a specific meter and should be chanted as they were conceived - no ifs and buts.
Mantra chanting requires accurate pronunciation (mispronunciation means no/incorrect results delivered), where as Sloka chanting need not be very accurate (yet one must put in efforts to make it reasonably accurate). For mantra chanting, it is not necessary to know the meaning because the effect/vibrations created from mantra chanting will deliver the intended results, whereas when one does Sloka chanting, knowing the meaning will have a tremendous effect on the person.
Vedas were not written or spoken by God. This knowledge emanated from God and originated in the heart of the Rishis (Sages) and they in turn composed the divine rhymes using Sanskrit ("the language of the Gods"); these Mantras are a combination of mystical words and sound waves. Mantra is recited to relax the mind and to fulfill the desired work. Mantra is an invocation or a mystical formula, which aids the person to realize the self and attain bliss and ultimate fulfillment.
The sounds involved in a Mantra are themselves significant for they generate in the individual, an unusual mystic power. The sounds activate the energy centers of our body, thereby causing changes to our subtle body (i.e., our senses and our mind). Mantras produce a set of vibrations in the surrounding atmosphere & its force depends on the attitude of the person as well as the intensity of concentration. For this reason, great emphasis is put on correct pronunciation. Mantras are used in our spiritual traditions to divert the mind from basic instinctual desires or material inclinations, by focusing the mind on a spiritual idea, such as "I am a manifestation of the divine consciousness".