In the first sentence, a refers to a general noun, frog. It could be any frog.

In the second sentence, an refers to the kind of frog which Robin saw.

In the third sentence, the refers to a specific noun, frog. It is the same frog which both Robin and Margie saw.

A, an, and the are called articles. A and an are called indefinite articles as they point out an indefinite noun.

A is used before a noun beginning with a consonant and an is used with a noun beginning with a vowel.

The is called a definite article. It points out a definite noun.

The articles a, an, and the in the above sentences are called limiting adjectives or determiners. Limiting adjectives specify or limit the noun or pronoun referred to.

Cardinal numbers such as one, two, and thirty and ordinals such as third, fourth, and eleventh are also limiting adjectives. They tell the number or position of a noun or a pronoun.