Tense is the time described by a verb, shown by its grammatical form. There are three main tenses in English :

1) Present Tense - things that happen in the present.

For eg) she goes to the school.

2) Past Tense - things that have already happened in the past.

Eg) she went to the school.

3) Future Tense - things that are about to happen in the future.

Eg) she will go to school tomorrow.

These 3 tenses are further divided into

a) Simple Present Tense

b) Present Continuous Tense

c) Present Perfect Tense

d) Present Perfect Continuous Tense

and similarly 4 for past and future tenses.

Thank-you.