Catch crops and cover crops have the same purpose (They ensure continuous soil cover) the only difference is that catch crops grow for a shorter period than cover crops.
Catch crops - Are grown from harvest of previous crop until seeding of new winter crop.
Cover crops (Winter)
Grows from harvest of previous crops until it dies before or during winter.
A cover crop will ONLY be seeded before a spring crop.
Cover crops (Spring)
Grow from harvest of previous crop until seeding of a new spring crop.
If a field with cover crops is being grazed, the quality of the remaining green cover will determine the amount of generated certificates.
If you want to read more about cover and catch crops, see this article: