In culinary terms, aioli is an emulsion which is a mixture of two or more ingredients that normally don't coexist. Oil and vinegar or oil and lemon juice are examples of emulsions. In the case of aioli, it is the egg yolk and olive oil that wouldn't normally coexist. But, unlike the previous examples, aioli is a permanent emulsion - it won't separate after time whereas you will always have to shake your salad dressings before each use.
For a relatively quick (it does require a little prep time) and no fuss version using ready made mayonnaise and roasted garlic, check out my Cheater's Aioli recipe.
If you are looking to make your aioli from scratch, then a great article is 'How To Make Mayonnaise, Aioli and Their Derivatives'. Not only will you find out how to make aioli and mayonnaise from scratch, there are also some recipes for salad dressings such as Thousand Island, Blue Cheese, and Ranch Dressing.
Of course, you can also never go wrong with a Jamie Oliver Aioli Recipe.
Photo used under Creative Commons from jules:stonesoup