It's a sponge cake with buttercream icing and sometimes a jam layer.
American weddings don't have two-tiered, 'evening do's so often the wedding will take place and then the cocktail hour is to tide people over until the dinner. If you're invited, you are expected to go to the whole thing.
So the cocktail hour happens whilst the couple gets photos done and all that stuff and then you go into the reception for the meal and then the dance.
Or, for those who have smaller budget or are religious, there is wedding and then cake and punch (no alcohol) reception in the church hall and then you go home.
But if you are invited you are invited to all of it.
They can be VERY flexible, too, because in most states you can marry just about anywhere.
I attended a fab wedding on top of a mountain (that was road accessible for older people) followed by a BBQ where the bridge and groom laid on meat and drink and everyone brought a dish, and mates with a band played, and then a campout.