Subsistence farming is growing food for your own and your family’s direct consumption. Like a backyard vegetable garden, but with fruit, starch crops, and animals as well.
Intensive agriculture is anything that really works the land hard. The inputs cost money, so it is generally only done for profit. Intensive livestock farming is stuff like feedlot cattle, most piggeries, indoor hens (battery or barn), and even most ‘free range’ hens - anything where the feed is grown elsewhere and brought to the animals because the land they are being kept on isn't big enough to grow all of their food in the paddock and let them graze it for themselves.
Intensive plant farming is not as clearly defined, but generally market gardens would be intensive, and a few thousand acres of wheat is extensive.