This is one instance of the distinction between regular and irregular inflection, which can also apply to other word classes, such as nouns and adjectives.
Most inflectional irregularities arise as a result of series of fairly uniform historical changes so forms that appear to be irregular from a synchronic (contemporary) point of view may be seen as following more regular patterns when the verbs are analyzed from a diachronic (historical linguistic) viewpoint.
Sometimes the result of multiple conditional and selective historical sound changes is to leave certain words following a practically unpredictable pattern.
French verbs, for example, follow different patterns depending on whether their infinitive ends in -er, -ir or -re (complicated slightly by certain rules of spelling).
A verb which does not follow the expected pattern based on the form of its infinitive is considered irregular.
In some languages, however, verbs may be considered regular even if the specification of one of their forms is not sufficient to predict all of the rest; they have more than one principal part.
To some extent it may be a matter of convention or subjective preference to state whether a verb is regular or irregular.
In terms of pronunciation, these make their past forms in the regular way, by adding the /d/ sound.
In studies of first language acquisition (where the aim is to establish how the human brain processes its native language), one debate among 20th-century linguists revolved around whether small children learn all verb forms as separate pieces of vocabulary or whether they deduce forms by the application of rules.
In fact, children often use the most common irregular verbs correctly in their earliest utterances but then switch to incorrect regular forms for a time when they begin to operate systematically.
When languages are being compared informally, one of the few quantitative statistics which are sometimes cited is the number of irregular verbs.
These counts are not particularly accurate for a wide variety of reasons, and academic linguists are reluctant to cite them.
Particular articles include, for example: Some grammatical information relating to specific verbs in various languages can also be found in Wiktionary.