The two main parts of a syllable are the onset and the rhyme (sometimes spelled rime). The rhyme is composed of a nucleus and an optional coda and is the part of a syllable that is used in poetry to form rhymes. The nucleus, as its name indicates, is the core part of a syllable and is always a vowel. Syllables can also appear without an onset; so the smallest possible syllable would be made up of a nucleus only.
onset = optional start of syllable = Consonant
nucleus = mandatory core of syllable = Vowel
coda = optional end of syllable = Consonant
Nucleus only: a
Onset and nucleus: la
Onset, nucleus and coda: lab
nucleus and coda: at