Thankfully Theo doesn't seem to vomit it back up again! My previous Abyssinian was obsessed with eating grass but would also always throw it up again a little while later (usually once he was indoors, and usually on the carpet).
It was just grass from the lawn out front. There do seem to be some different varieties growing there though, and both kittens seem to prefer a particular type (with a fairly soft broad blade). I think the cat grass is usually a cereal grass, like rye or oats.