A hypothesis is a tentative statement about the relationship between two or more variables. A hypothesis is a specific, testable prediction about what you expect to happen in your study. For example, a study designed to look at the relationship between sleep deprivation and test performance might have a hypothesis that states, ""This study is designed to assess the hypothesis that sleep deprived people will perform worse on a test than individuals who are not sleep deprived.""

Hypothesis is of two types:

1. The null hypothesis, H0, plays a central role in statistical hypothesis testing: It is the hypothesis that is assumed to be true and formally tested, it is the hypothesis that determines the sampling distribution to be employed, and it is the hypothesis about which the final decision to ""reject"" or ""retain"" is made
2. The alternative hypothesis, H1, specifies the alternative population condition that is “supported"" or “asserted"" upon rejection of H0. H1 typically reflects the underlying research hypothesis of the investigator. If the interest is only in one direction it is called directional alternative hypothesis. Else, if interest is in both directions it is called  non-directional alternative hypothesis.

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Sources:
http://psychology.about.com/od/hindex/g/hypothesis.htm
Fundamentals of Statistical reasoning in Education, Theodore Coladarci, Casey D. Cobb, Edward W. Minium and Robert C. Clarke (Click for eBook)