Truffle oil is a good quality olive oil infused with the aroma of truffles. Truffles are a type of fungus that live in harmony with oak trees, and other deciduous trees, buried underneath the soil. They can sell for up to $2000 per pound! Apparently, truffles emit a scent very similar to the male pig sex hormone, which explains why female pigs were used to find these prized morsels (nowadays they use dogs because pigs often ate the truffle). Truffles and truffle oil are very pungent and pack quite the punch so a little goes a long way (a dish can be easily ruined by using too much truffle); it can be used on pasta, salad, risotto, foie gras, poultry, eggs, potatoes, etc.
Truffle oil should be stored in a dark cool place and used within 3 to 4 months. I can't see myself using the entire bottle in such a short amount of time so after opening, I closed it tightly, wrapped some plastic wrap around the top, and placed it in the fridge. It might turn cloudy after a while but that will dissipate when it warms up to room temp (in a dark place).
The truffle flavor is very hard to describe; to me it is unlike anything I have ever tasted and has a taste of its own. It didn't remind me of walnuts or mushrooms, like others have suggested. Honestly it was a bit... strange at first but the flavor grew on me. It imparts so much depth and body to whatever food it mingles with but it's not something I would use regularly.