In Gardening, What Are Companion Plants?

Using companion plants is a technique used by gardeners to ensure that pests are controlled, without needing to use pesticides.
This is particularly useful for organic gardeners for whom pests can be a real problem, but only a limited range of natural insecticides are available.
The process involves planting different types of plants together, one of which will help protect the other.  So for example, onions grown amongst carrots will help deter carrot fly.  To reduce greenfly, garlic should be grown alongside roses.  Lavender near roses can also prevent aphids attacking the roses.  Whitefly can ruin tomatoes, but they can be kept at bay by planting some French marigolds nearby.  
There are many other companion 'buddy' plants and most gardening books offer some sort of advice on this gardening technique.
None of these techniques are 100% reliable, but what they can do is offer some protection to the plants-and some hope to gardners who dislike using chemicals within the garden, particularly on fruit and vegetables.