Distributions and formuli
The number of heads in a 100 flips follows the binomial distribution. Most statistics books have a discussion of the binomial formula, so we'll skip it here. Below you see the distribution of 1040 sets of 100 flips, with the normal curve overlaid in black.
What does this mean about your data?
More formally, the theoretical distribution has a mean of 50 and standard deviation of 5. So this means that 68% of the time the number of heads will vary between 45 - 55 (within one standard deviation), and 95% of the time it will vary between 41 -59 (1.96 standard deviations).
Note that even with 1,040 samples of 100 flips, the above distribution is not a perfectly normal -- there were too many samples with more than 50 heads, creating a negative skew. Why do you suppose that is? (Write your answer below)