In general, these verbs used to end in a g, k or ch /tʃ/ sound. In some of the modern forms of these words, this sound has changed, such as in "buy". The "t" of the ending is just a form of the past tense suffix, as in "burnt" or "learnt". Before this -t, a g, k or ch sound often alternated with a kh-like sound /x/ in older forms of English (this sound is like the ending sound of "Bach" in German); this sound is produced in the same place in the mouth as the k and g sounds. That is what the "gh" represents; this sound was still pronounced relatively recently in English, for example in Chaucer's time. Before this kh-sound, an "n" was regularly dropped, which is why you don't see the "n" of "bring" and "think" in their past tense forms.
The reason for the "u" before the "gh" is that the kh-sound influenced the vowel before it. You'll notice that the sequences "agh" and "ogh" simply don't appear in the spelling of Modern English. (Some of these words also had more complicated vowel changes in the past tense from earlier on in their history.)
So that's the reason why the sequence "-ught" shows up in all these words: the "t" is the past tense ending, the "gh" is a remnant of what became of the last consonant of these verb's present-tense forms, and the "u" reflects the effect the formerly-pronounced "gh" sound had on the preceding vowel.
You can see that "fight" has the same ending for the present and past; it belongs to the much larger category of English verbs that have a different vowel in the past tense and no past tense suffix, like "ride" and "rode".