Oils and resins
There are fatty oils, and there are essential oils, and there are resins.
The difference between fatty oils and essential oils is, if you put a drop of a fat oil onto a piece of paper, the stain will sit there for as long as the paper exists.
If you put a drop of an essential oil onto that same piece of paper, it'll evaporate off, leaving nothing except possible a faint trace of color outline, and an exquisite scent (or stink, depending).
Seeds which contain fat oils are processed into oils - the el cheapo cooking oils (they use heat, I think, to get the oils out), or the more expensive cold-pressed cooking oils. El cheapo oils go rancid at the drop of a hat, but some cold-pressed oils do, too. Those cold-pressed oils which contain loads of vitamin E go rancid slower than the rest - it'll take a year or two for them to go off. Me, I use mostly certified organic cold-pressed cooking oils, both in my cooking and in my herbal salves and oils.
It's possible to make your own essential oils, but generally, you need a few hundred kg (or pounds) of fresh herb to get a liter (or pint) of essential oil. Which is why those oils are so expensive. Because they're so expensive, they're of course also adulterated ... I prefer using my essential oils as integrated parts of my herbs, not as extracted drops.
Plant families with lots of species with essential oils are, among others: the mint family (Lamiaceae or Labiatae), the citrus or rue family (Rutaceae), the carrot family (Apiaceae or Umbelliferae), the pine family (Pinaceae), and I'm sure I've forgotten one or the other large group.
What do these essential oils do? Some of them are good for gut or menstrual cramps, others are really good germ killers. Of those plants which work (partly) because of essential oils, I like chamomile, yarrow, the true mints (Mentha spp.), most of the leaf and seed spices, lavender, and rose, just to name a few.
Resins are (usually) concentrated essential oils, and the resins from edible conifers are diuretic because they're irritating to the kidneys.
I don't use all that many resinous plants. Sure, I tell people that they can chew on a juniper berry as a bitter, and I make a spruce shoot syrup in early summer, but those are side comments, they're not mainline herbal use - at least for me.
Link: Essential oils in the medicinal herbfaq
Other entries: Constituents - Herbal salves - Using juniper berries - Carrot Family Carminatives