A salad is a dish consisting of pieces of food in a mixture, sometimes with at least one raw ingredient. It is often dressed, and is typically served at room temperature or chilled, though some (such as south German potato salad, or chicken salad) can be served warm.
Garden salads use a base of leafy greens such as lettuce, arugula/rocket, kale or spinach; they are common enough that the word salad alone often refers specifically to garden salads. Other types include bean salad, tuna salad, fattoush, Greek salad (vegetable-based, but without leafy greens), and sōmen salad (a noodle-based salad). The sauce used to flavor a salad is generally called a salad dressing; most salad dressings are based on either a mixture of oil and vinegar or a fermented milk product like kefir.
Salads may be served at any point during a meal:

Appetizer salads—light, smaller-portion salads served as the first course of the meal.
Side salads—to accompany the main course as a side dish, examples include potato salad and Caesar salad.
Main course salads—usually containing a portion of a high-protein foods, such as meat, fish, eggs, legumes, or cheese.
Dessert salads—sweet versions containing fruit, gelatin, sweeteners or whipped cream.