Too means "also" and is generally used at the end of a sentence. "Too" also indicates too much of a particular quality.
Examples:
That car is too expensive for me!
I'd love to come to the party, too.
Two is the written form of the number 2.
Examples:
There are two applicants for the job.
She has two cats.
To is generally used as a preposition. It is also used as part of the infinitive form of verbs.
Examples:
I gave the book to him.
The verb "to understand" is irregular.