The A is a secondary part of the sentence which qualifies a noun, a pronoun, or any other part of speech that has a nominal character. It doesn't agree with the word it modifies in number, case, or gender. The A can be expressed by:
1) An adjective (A nice person);
2) A pronoun (possessive, defining, demonstrative, interrogative, relative) (Their song);
3) A numeral (cardinal or ordinal) (First place);
4) A noun in the common case (City legend) and genitive case (His friend's mother); 5) A prepositional phrase (A letter from my sister);
6) An adverb in pre-position (After shock) and post-position (The room below);
7) Participles 1&2 or a participial phrase (Painted house);
8) A prepositional phrase or a prepositional construction with a gerund (I like the idea of visiting you);
9) An infinitive, an infinitive phrase or construction (always used in post-position) (It's my chance to earn a lot); 10) Quotation groups (I don't like his know-it-all tone).
An apposition is a special kind of A which is expressed by a noun which characterizes or explains the word modified by giving the person or thing another name. The close apposition is not separated by commas and stands in close connection with the word modified.
These word-groups generally consist either of the name of a person and a noun denoting relationship, or a geographical name and some common noun (the first component is a common noun in apposition) (Uncle Tom, the city of London). The loose or detached apposition is always separated by commas and has a stress of its own (Mary, my groupmate, is a nice person).