Vegetable oil is a broad term for a category of oils pressed from the seeds, nuts, grains, or fruits of plants. With the exception of specialty oils (such as toasted sesame or nut oils), vegetable oils are refined and filtered to create clear, relatively neutral-tasting oils to be used for cooking, frying, and baking. Examples include corn, canola oil, peanut, grapeseed. You will also find bottles simply called vegetable oil, which often contains a blend of different oils.
Light olive oil can often substitute in vinaigrettes and marinades (extra-virgin may offer too much flavor) and for cooking. Butter may also be substituted in the frying pan, though it burns more easily than oil; clarified butter would be a better choice.
When choosing an oil for dressings, marinades, and baking, the only real criteria are taste and health preferences. For sautéing and frying, however, you'll do best using an oil with a high smoke point. Your better bets for serious frying include grapeseed, peanut, corn, and safflower oils.
Keep capped and away from heat.