in the germanic languages, a strong verb is a verb that marks its past tense by means of changes to the stem vowel (ablaut). the majority of the remaining verbs form the past tense by means of a dental suffix (e.g. -ed in english), and are known as weak verbs.
in modern english, strong verbs include sing (present i sing, past i sang, past participle i have sung) and drive (present i drive, past i drove, past participle i have driven), as opposed to weak verbs such as open (present i open, past i opened, past participle i have opened). not all verbs with a change in the stem vowel are strong verbs, however; they may also be irregular weak verbs such as bring, brought, brought or keep, kept, kept. the key distinction is that most strong verbs have their origin in the earliest sound system of proto-indo-european, whereas weak verbs use a dental ending (in english usually -ed or -t) that developed later with the branching off of the proto-germanic. as in english, in all germanic languages weak verbs outnumber strong verbs.
the "strong" vs. "weak" terminology was coined by the german philologist jacob grimm in the 1800s, and the terms "strong verb" and "weak verb" are direct translations of the original german terms "starkes verb" and "schwaches verb".