Perhaps the easiest way to explain marital property is to say that it is not separate property.
Separate property
Separate property is the property you had when you married; property you inherit during the marriage; and property you receive as a gift during the marriage.
Marital property
Marital property is property you obtained during the marriage except gifts and inheritances. It doesn’t matter whose name it is titled in, who paid for it, or who uses it: if you got it while you were married and it wasn’t a gift or inheritance, it’s marital property.
Mixed property
There are ways in which separate property can either become marital property, or be treated by a court as having a marital component or interest.
One common pattern is when the value of property is increased during the marriage by marital effort–work by either or both of the parties–or the use of marital money (which includes the wages of either party). Suppose the wife owns a house when the parties marry: since it was hers’ before the marriage, it is separate property. But now suppose that during the marriage, the parties increase the value of the house by adding an addition. The parties buy supplies using money they’ve each earned during the marriage. Some of the work is done by a contractor, and some by the parties themselves (or even just one of them). The increase in the value of the house attributable to the addition–not the value of the entire house, just the increase due to the addition–is marital property and included in the marital estate. Note that if the parties had not added the addition but the value of the house increased just because the price of houses increased, that increase is not marital. It’s the increase in value because of marital effort or marital money that is marital.
Another common pattern is when the net value of the property is increased during the marriage because marital money was used to reduce the debt: when the equity increased because they used their wages during the marriage to pay down the mortgage. As with the previous example, an increase in equity just because the price of houses increased is not marital. It’s the increase in net value because marital money was used to pay down the debt that is marital.