A probability distribution is a table or an equation that shows each outcome of random variable with its probability of occurrence.
If we toss a coin and count the number of head, the table below will be the probability distribution.
| Number of heads | Probability |
| 0 | 0.5 |
| 1 | 0.5 |
| Sum | 1.0 |
If we toss two coins and count the number of heads, the table below will be the probability distribution.
| Number of heads | Probability |
| 0 | 0.25 |
| 1 | 0.50 |
| 2 | 0.25 |
| Sum | 1.00 |
If we toss three coins and count the number of heads, the table below will be the probability distribution.
| Number of heads | Probability | |
| 0 | 0.125 | |
| 1 | 0.375 | |
| 2 | 0.375 | |
| 3 | 0.125 | |
| Sum | 1.000 |