From a phonetic standpoint, diphthongs come in (at least) two flavors, rising diphthongs and falling diphthongs. A rising diphthong moves from lower in the vowel space to higher. Since the high vowels like /u/ and /i/ tend to form glides (and low vowels tend not to), a rising diphthong will generally
wind up having a full vowel followed by a glide (an "off-glide") like /y/ or /w/. Words with these kinds of diphthongs include "buy,"
"toy," "play," and even (as most English-speakers say it anyway), "main."
Falling diphthongs go the other way, from high in vowel-space to lower. This means they tend to start with a glide and move to a full
vowel, like in "swell," "yes," "you," "yodel."
Many English-speakers tend to pronounce all long "o" sounds as diphthongs.