Null Hypothesis is one of the confusing terms that most students really struggle with, so get comfy, grab a cup of coffee, and we'll get through it together....
The simplistic definition is that the null hypothesis is the opposite of the hypothesis being tested. The researcher suspects the hypothesis to be true (and thus is doing research to support the hypothesis), but the null hypothesis is the hypothesis the researcher tries to disprove. The researcher never proves or accepts the null hypothesis, but can only reject it or not reject it. Confused? How about an example?
Hypothesis: Roses exhibit greater rate of growth when planted in soil rather than compost.
Null Hypothesis: Roses do not exhibit greater rate of growth when planted in soil rather than compost.
Ok, these are lame, but they make the point. Let's say the researcher collects and analyzes the data, and the results are statistically significant at the 99% level (the data show that roses do grow better in soil with 99% confidence) - in this case, the researcher would reject the null hypothesis and accept the hypothesis.