Some effects of repetition include pattern recognition, habit formation, memorization, familiarity and comprehension. In general, the human brain is naturally hard-wired to make associations through repetition. These associations are not always good, however. Sometimes, people grow to dislike things as a result of too much repetition.
The brain typically retains most pieces of information between 10 seconds and one minute. This is what is known as short-term memory, and it is a natural function of the mind's prioritization method.
As information is received, it is evaluated. When people say, do or see things over and over, their brains start to recognize a pattern and begin assigning that information higher priority. Their brains are also able to assign the information meaning and place it into a context, which also helps people remember things. Eventually, something is committed to long-term memory after it is repeated enough times, and the brain comes to realize that it is important. This process explains why information or processes are often repeated many times when students are learning something new in school. It's also the theory behind the importance of practicing sports or playing musical instruments. The more times people practice, thus committing something to memory, the better they become at it.