To iterate is to say or perform again or repeat. To reiterate= to say or do again repeatedly.

Technically, reiteration and repetition are the same and can be used interchangeably, as Jura says. Usually, though, 'repetition' is used when something is repeated many times. "He learned through repetition." You would almost never hear "He learned through reiteration.", even though it is correct. In actual usage, the verb 'reiterate' is usually used when something is said once again for emphasis. "I reiterate. This highway should NOT be built near our neighborhood!" You could use "I repeat...." in the above example. Both would be acceptable. However, you would almost never hear "He reiterated the exercise." 'Reiterate' is usually used for something verbal. For anything else, 'repeat' is almost always used, even though 'reiterate' would be correct.