What is the difference between species diversity and species evenness ?

Species richness is the number of species found in an area- you simply count the number of different species. Species evenness is how evenly spread the population is across the species in an area- if you have 100 cats and 105 dogs, the population has high evenness but if you had 200 cats and 5 dogs, even though the number of animals is the same, the population is not very even.