Articles are words (a, an, or the) that appear before certain nouns they are modifying, like adjectives. They are determiners and can be either definite (the) or indefinite (a/an). Definite articles like the refer to specific nouns while indefinite articles like a/an refer to more general or non-specific nouns.  

Articles Overview: this UNC-Chapel Hill resource provides relevant definitions, guidelines, and a helpful flowchart for choosing what article—if necessary—to use.