It's easy for me to imagine the difference, but hard for me to conceptualize it. I guess one involves two vowels and the other involves a consonant, right? Am I on the right track, or is there a more precise definition?
If a language has a sequence of two vocoids, and one of them is high, there will be nothing in the signal to tell you whether you are dealing with a vowel+glide sequence or a vowel+vowel sequence. Three common sources of supplementary information (in decreasing order of empirical weight) are:
1) Cases where there is a lexical contrast between the vowel+vowel and the vowel+glide sequence. In Vietnamese, for example, there are lexical contrasts where the relevant difference is /-ai/ vs. /-aj/, or /-ou/ vs. /-ow/.
2) Morpho-phonological alternations or selectional restrictions that are sensitive to a vowel-consonant distinction. Two examples are given in Dixon (2010:196--9).
3) Phonotactic considerations. Is a statement of the overall phonotactic patterns of the language simplified by choosing one analysis over the other?
There will also be cases where no evidence is available. In these cases it will have to be admitted that there are no language-internal grounds for classifying the pattern in question.
Dixion, R.M.W. (2010) Basic Linguistic Thoery, vol.1. OUP
The simplest distinction is
a glide is a single phoneme that is somewhere in the middle of the continuum between consonant and vowel, but is non-syllabic (by itself).
a diphthong is a sequence of two vowels, where one of them is often articulated just like a glide. It is questionable whether this two-vowel combination is a single phoneme or two separate phonemes in sequence, but there are two 'things', one of which is glide-like and the other a full vowel.
I hesitate to call the glide-like phone in a diphthong a full fledged glide only out of tradition, even though they are seemingly identical articulations.
The primary difference then is that a diphthong includes a glide as one of two constituent parts.
Rather than a "vowel + consonant", a glide is a semivowel (it's a synonym). I posted once about them on here, for the question "Are there semivowels besides /w/ and /j/ and which are most common?". You can read more in the links
A diphthong is obtained by the combination of two vowels occurring in the same syllable. This combination varies according to the language and the rules that belong to the language itself.
- In Spanish for example, a diphthong is obtained by joining a closed vowel (i, u) with an open vowel (a, e, o).
- In Italian it's obtained by joining the vowels (i, u) in unstressed position to other vowels in stressed position, or by joining the same vowels, (/i/ and /u/ together), where one of them — in this case — can be in the stressed position.
And so on... You can read about more languages by clicking on the link.
In The Sound Pattern of English, SPE, a feature +/- syllabic, attributed to work by C.-J. Bailey, is used to distinguish [+syll] sounds which make for a syllable from [-syll] sounds, which do not make for a syllable. Or at least this is my interpretation of SPE. "Make for a syllable" means that the number of [+syll] sounds in a form will always be the same as the number of syllables in the pronunciation of the form.
Then, using this feature, glides are distinguished as [-syll], and diphthongs are characterized as combinations of syllabic and non-syllabic sounds. Rising diphthongs are [-syll][+syll], and falling diphthongs are [+syll][-syll]. In this conception, the difference between glide and diphthong is phonetic, and concerns whether a form has a syllable or not. A diphthong has a syllabic sound, but a glide does not.