What's The Difference Between Pecan And Walnut?

The difference between a walnut and a pecan nut is basically their shape, species taste, consistency, cost, and their health benefits.
Both are from the Juglandaceae species, which also includes hickories, are grown on hardwood trees around the world.
The English walnut and Persian black walnuts are larger than pecans and taste sweeter and are widely used in cooking and baking.
Walnuts are usually broader with more texture and taste than the pecan as so tend to cost more.
Pecans are used les sin cooking although are an excellent source of antioxidants, unsaturated fats and omega-6 fatty acids, good for reducing cholesterol, especially the bad category of LDl cholesterol.
Tests on rats have shown that pecans could delay the ageing process, especially muscle degeneration
American president Thomas Jefferson planted pecan trees, Carya illinoinensis (Illinois nuts), in his nut orchard at his home, Monticello, in Virginia and gave George Washington some as a present. 
The walnut contains glyceryl triacylates, omega 6 and n-3 fatty acids, and so are also good for reducing cholesterol.
The Journal of the American College of Nutrition concluded in 2010 that walnuts and walnut oil improve reaction to stress.
Another good source of protein, raw walnuts are also thought to contain high levels of antioxidants, and can help in reducing obesity, although roasting walnuts can reduce this effectiveness.

Pretty much, although walnuts have a slight bitter and oily after taste, whereas pecan nuts have a much woodier, nuttier taste and seems light like a roasted almond

Yes it is actually apart of the same family

The Pecan nut is a source of ellagic acid, and they also have anti-cancer effects, They are a good low fat source of vitamin E and if eaten correctly can lead to lower cholesterol levels. Studies have shown that they can lower the risk of Gallstones.

The nut comes from the Pecan tree which is a species of hickory that is native to the south eastern part of North America. The tree grows 26 – 40 metres in height and is deciduous.

The nuts have a rich buttery flavour. They can be eaten fresh or used in cooking. One of the most common uses is as an ingredient in the Pecan Pie, a traditional recipe from the south of USA.

The wood of the Pecan tree is also used to make furniture and hardwood flooring as well as providing flavouring fuel for smoking meats.

It is not related to the walnut.