Beginning at the date of marriage, every dollar earned during the marriage is marital funds. If these funds are put into a checking/savings account, retirement account, investment account, etc. that account now becomes a marital asset. The money used to fund that account is marital money so the account becomes marital as well. If there is an account that a spouse opened prior to the marriage and there are no contributions whatsoever during the marriage, that account will remain separate property.
This concept is important to understand for the equitable distribution portion of a divorce. When determining how to split the marital assets, this concept comes into play and account statements must be produced if a spouse is claiming a certain account is separate property; it is up to that spouse to prove their separate property claim.