Strong verbs are those ones that change the vowel during conjugation (see chapter 5). One could discuss at length why they are called strong, but it doesn't change the point that they are called that way.

There are three types of strong verbs
a) strong verbs without a change in the vowel present tense, but change in past tense
b) strong verbs with a change of vowel in present tense and in imperfect

We'll start with the first group.