It depends. If you are a Maryland resident, you are taxed in Maryland on all of your income. If some, even all of your income is earned in Delaware, you are required to report it. In this case, you would file a Delaware nonresident return and a Maryland resident return. While you are taxed in both states on the income, Maryland will give you a credit for the taxes you pay to Delaware so that you are not double-taxed.
If you are a Delaware resident with some income from Maryland, Maryland will not tax your Delaware income. They may, however, factor it in in determining your Maryland tax. What they do is calculate your tax based on your entire income, and then tax the percentage of the income that is earned in Maryland. In this case, you prepare a Maryland nonresident return and a Delaware resident return.
Regardless of the situation, if you are a resident in one state all year you are taxed on all of your income regardless of where you earn it. Because of this, you want to prepare any nonresident return first so that the correct credits may be applied on the resident return. Here is a TurboTax FAQ with more information: https://ttlc.intuit.com/replies/3302052
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