Vegetables are a culinary class of food. They are parts of plants which are usually not eaten eaten raw, ground up, or on their own and which tend to be savory (or at least not sweet). Unlike "fruit," "vegetable" has no botanical meaning. It is a purely culinary term. Most dictionaries define it something like "a part of a plant used for food," which isn't very helpful. Obviously apples, oranges, and almonds are not vegetables. A better definition is provided by Wiktionary as:
The edible part of [a plant raised for some edible part of it, such as the leaves, roots, fruit or flowers, but excluding any plant considered to be a fruit, grain, herb, or spice in the culinary sense.]
Again, this is purely a culinary term. As such, it is sometimes hard to draw firm lines where one type of food ends and where another being. What is and is not a vegetable is a rather vague question. It's basically any plant that is not a fruit. This makes it easy to include any part of a plant that is eaten other than the botanical fruit of the plant, but that is not all vegetables. Some things are botanical fruits but still vegetables, and saying what is and what is not a culinary fruit is just the flip-side of the same question.
It's hard to pin a firm definition on a purely cultural construct, but some of the common identifying traits of vegetables are:
- Typically, vegetables are much less sweet and juicy than fruits.
- Vegetables are typically eaten as part of a main course with other vegetables and/or meat.
- Vegetables are often less colorful and are most commonly just green.
- Vegetables often need to be cooked before eating.
- Vegetables often grow close to or under the ground.
- Vegetables typically grow on annual plants.
Weighing these factors can help identify what is a vegetable, but few are hard and fast rules. You ultimately have to just figure out and go with the cultural determinations. This site can hopefully help with understanding those determinations and with drawing that line between fruit and vegetable, although I will be honest and admit that there are some things like avocados that I'm not even sure about. I always welcome feedback.