AnaI, is it possible you are confusing essential oils and infused oils?
Essential oils are a volitile part of plants, often responsible for their flavor and aroma. As Luc explained, they are extracted from the plants using various distilation methods.
This is not, btw, a tabletop operation that you can do at home.
Infused oils, on the other hand, are often used for culinary purposes. For those, a flavoring agent is mixed with an oil and allowed to sit for various time periods. The oil extacts the flavors, which get infused through it.
For instance, I recently made a mint-infused oilive oil for one of Nino Graziano's great recipes. This is one of his basic oil infusions:
1 dl (scant 1.2 cup) extra virgin olive oil
1 bunch fresh mint
Puree the mint leaes with the oil in a food processor or blender. Strain through a fine-mesh sieve (discarding the solids) and reserve the oil.
Note that mint infuses quite readily, so it doesn't have to sit for an length of time. On the other hand, a tangerine-infused oil he also makes has to macerate for 6 days.
Infused oils are also an integral part of herbal medicine. The basic balm I make, for instance, uses six different herbs, each infused seperately into the oil. But the process is the same, whether you are making medicine or making dinner.
They have taken the oath of the brother in blood, in leavened bread and salt. Rudyard Kipling