Here's a little table of a few different poultry breeds and what they are probably best suited to in respect to laying eggs and providing meat. Note that turkey is not listed as their conversion of feed to lay rate for eggs makes them an unviable egg producer, though their eggs are reputed to be quite alright!
Advantages of cross breeding
Most commercial producers use cross bred stock. The reason this is the case is duet to an occurrence known as "heterosis" or hybrid vigour. As a result of cross breeding from two different purebreds such as a Rhode Island Red crossed with a White Leghorn, the offspring are often genetically superior and as a result of hybrid vigour a number of factors such as disease resistance, egg laying rate, behaviour, feed to meat conversion and general health can be seen to be markedly better. Most people tend to keep purebreds for ornamental purposes and as show birds rather than for their eggs and meat.
The other bonus for hybridising pure breds is that you can choose the positives of each breed that can then be passed to the offspring, which gives a farmer greater control in finding an appropriate breed for their circumstances, be it disease resistance, certain egg size, output or egg aesthetics.
Pure & Cross
Poultry is made up of a number of families of birds that include chickens, turkey, ducks, geese and quails. A pure bred bird arises from breeding two poultry of matching genetic type who are of pure bred origin, therefore giving birth to another pure bred bird of that type. Examples of pure bred varieties are from Australia are Australorp, Australian Game, Australian Langshan and Australian Pit Game.
Cross bred varieties arise from crossing two different breeds to create a hybrid bird. The Isa Brown chicken is a prime example. Others include Daisy Belle, Cream Legbar, Cornish Game Bird and Orpington.
The breeds can also be broken into categories of "light", "dual purpose" or "table" birds. Some examples of chicken breeds that fall into these categories are :
Light : Australorps (good egg producers)
Dual Purpose : Rhode Island Red (good layer of large eggs, also good as parent stock for hybrids)
Table Birds : Indian Game aka Cornish (slow growing, poor egg layer but extremely large chest and extensive flesh)