The law says that Marital Property is any asset that comes into the marriage from the date of marriage until the date of separation (except for inheritances and a gift from a third party to one of the spouses which are non-marital). This means then that the assets you bring with you to the marriage will remain your separate, non-marital or pre-marital asset.
But, any increase in value of the pre-marital asset is marital. So, assume you come to the marriage with 100 shares of Disney stock and during the marriage the stock splits and doubles and increases in value by an additional 100 shares. Those additional 100 shares are marital property. Or suppose you own a home on the date of marriage and the home increases in value. Again, that increase in value of your pre-marital asset is marital property.
It is important that pre-marital and other non-marital assets remain individually titled. Once you put the non-marital asset into joint names, the law says you intended it to be a gift to the marriage as evidenced by the change in title. Turning an individually owned asset into a joint asset most often happens with a non-marital, pre-marital house when there is a refinance. During settlement, the non-ownership spouse can very easily be added to the title and the mortgage. Be aware.
To protect pre-marital and other non-marital property, I highly recommend that you enter into a Pre-Nuptial Agreement to define ownership rights in pre-marital property.