Strong verbs, weak verbs, and other categories

Verbs which demonstrate ablaut (like sing-sang-sung) are called strong verbs. Wikipedia:

In the Germanic languages, a strong verb is one which marks its past tense by means of ablaut. In English, these are verbs like sing, sang, sung. The term "strong verb" is a translation of German "starkes Verb", which was coined by the linguist Jacob Grimm and contrasts with the so-called "weak verb" ("schwaches Verb") which forms its past tense by means of a dental suffix.

Verbs whose past tense form and participial form make use of the regular -ed/-d/-t ending (like laugh-laughed-laughed) are called weak verbs, and a majority of those are also regular verbs. Wikipedia on weak verbs:

In Germanic languages, weak verbs are those verbs that form their preterites and past participles by means of a dental suffix, an inflection that contains a /t/ or /d/ sound or similar. (For comparative purposes we may refer to this generally as a dental, although in some of the languages, including most varieties of English, /t/ and /d/ are alveolar rather than dental consonants.) In all Germanic languages, the preterite and past participle forms of weak verbs are formed from the same stem.

Verbs like cut and put are categorized in this list of irregular verbs as "weak with assimilation of dentals". So their past tense and participial forms have an invisible -ed ending which has been linguistically assimilated (i.e. swallowed up into the t at the end of the word).

In Old English, strong verbs were those which changed the stressed vowel when forming the past tense and past participle. Ridan (to ride) became rad, riden; singan (to sing) became sang, sungen. Weak verbs kept the same stressed vowel, but added –d or –t for the past tense and past participle. So, lufian (to love) became lufode, lufod; locian (to look) became locode, locod.

Similar differences persist in modern English. The strong verbs ride and sing become rode, ridden and sang, sung. The weak verbs love and look become loved and looked in both the past tense and the past participle. Strong and weak verbs are more often called irregular and regular verbs now. Regular verbs are more numerous, but irregular verbs are among the most frequently used.