Once the parameters are set up, you can simulate multiple rounds of marking and recapturing with the same parameters. One graph shows the estimated population size with confidence bars for each sample taken, while another graph builds a histogram of estimated population sizes as the user takes more and more samples. There is good online help on each parameter for students that want additional information, and the author includes accompanying written material from his own use of the program if you want to adopt his labs. Though not flashy, the interface is functional and arranged well. The graphs make an effective demonstration of how different samples with exactly the same underlying dynamics can come up with different answers, and after many samples the histogram starts to look like a nice normal curve.
Students can also get a feel for how different parameters make population estimates better or worse.
I expect that this program would be most useful in upper level courses where students are taught enough about mark-recapture sampling that they have an understanding of how it works before using the program. The author also recommends using PopSize in conjunction with a field component where students actually do some sampling.