Population Size is the number of chromosomes in the population. Larger population sizes increase the amount of variation present in the population at the expense of requiring more fitness function evaluations. I have found that the best population size is dependent upon both the application and the length of the chromosome. For longer chromosomes or more challenging the optimization problems, I usually use of population size greater than 100, while for simpler problems a population size of 20 will lead to good results. Number of Generations is the maximum number of generations that will be performed.
Mutation Rate is the probability of mutation occurring.
Mutation is employed to give new information to the population. Large mutation rates increase the probability of destroying a good chromosome, but prevents premature convergence. The best mutation rate is application dependent and related to both the length of the chromosome and the size of the population. For most applications a mutation rate of 0.1 to 0.01 is employed. Crossover Rate is the probability of crossover reproduction being performed.