The easiest way to dress a salad is to simply add oil and vinegar or citrus juice directly in with the greens and season with salt and freshly ground black pepper. While this does season the greens, it requires a good technique and knowledge of the correct proportions. Most cooks prefer to make up a dressing beforehand so that its flavors are more thoroughly blended and so that the oil-acid balanced can be more easily adjusted if necessary. The dressing called vinaigrette, which is nothing but a combination of three to four (usually three) parts oil (usually olive) and one part acid (usually wine vinegar), seasoned with salt and pepper and often spiced with mustard and/or garlic.
While the ingredients in a vinaigrette are few and quite simple, the technique for combining them requires a bit of understanding. Left alone, the elements in a simple vinaigrette do not mix. They separate, making difficult to dress a salad evenly. The oil rises to the top and the vinegar, which behaves like water, sinks to the bottom. To create a dressing with an even distribution of oil and vinegar, the two elements must be combined in some sort of an emulsion, meaning that the two repellent ingredients become mixed. This is achieved by adding the oil to the vinegar and vigorously shaking or whisking until they temporarily blend together. In its basic form the vinaigrette will hold together long enough to pour over the salad. Many cooks, prefer a thicker, more stable emulsion that doesn't separate as quickly when left to stand, allowing time to attend to other parts of the meal. Adding other ingredients to the vinegar - such as mustard, garlic, onions, pureed vegetables, olive paste, or cream, etc to help thicken a salad dressing and make the emulsion more stable.
The best method to make a thick, well-emulsified vinaigrette is to first whisk together the vinegar or lemon juice and the seasonings (salt, minced shallots or other members of the onion tribe, and mustard) in a small bowl. Then slowly add the oil, drop by drop, whisking as you go until the dressing begins to thicken. Add the oil in more of a steady stream as the dressing becomes thicker. An alternative, and perhaps for convenient, technique is to mix the vinegar or lemon juice and seasoning in a blender or food processor then add the oil in a slow, steady stream with the machine running.
Vinaigrette can be stored, tightly covered, in the refrigerator for up to 2 weeks. Always whisk dressings briskly just before adding them to salads sot that the ingredients are well mixed and in balance, and don't add more vinaigrette than is necessary to lightly coat the salad.