A will covers all probate assets. Probate assets are those that are solely owned, and which do not have a designated beneficiary. For example, your prized fishing poles. Assuming you are the sole owner, those are probate assets, (i.e., they will have to go through the estate administration process). Therefore, the fishing poles are covered by the will, and will go to whomever you decide to bequeath them to. 

A home, however, that is jointly owned, is not a probate asset; it will not have to go through the estate administration process when the first owner passes away (it will have to when the last surviving owner passes away). This is not to say that you should not necessarily list it in your will. After all, perhaps the other owner will pass away first, leaving you as the sole owner of the home. In which case, it will be a probate asset. But, if you pass away first, and the joint owner is you wife, even if you leave the home to your son in your will, the home will go to your wife. 

Another non-probate asset is an IRA. Assuming the IRA has a designated beneficiary, then even if you decide to leave the IRA assets to someone else in your will, the will won’t control. 

As such, efficient estate planning goes beyond the scope of a mere will. It’s important to consider the totality of your assets, and structure your estate plan accordingly. Many a mistaken testator has left a non-probate asset to a beneficiary by will, while the hopeful beneficiary discovers, to late, that the will designation was meaningless.