The Korean alphabet is called Hangul, which also means a single syllable.

Each hangul (syllable) is composed of letters, which are called jamo.

The structure of a hangul is as follows: initial consonant, vowel, final consonant.

If the vowel is vertical, you put the initial consonant to its left and the final consonant below it:

난

(this syllable is composed of the initial consonant ㄴon the left, the horizontal vowel ㅏ and the final consonant ㄴ at the bottom)

If the vowel is horizontal, the initial consonant goes on top, the vowel is in the middle and the final consonant is at the bottom:

눈

(this syllable is composed of the initial consonant ㄴon top, the vertical vowel ㅜ in the middle and the final consonant ㄴ below)

Some syllables might not have the final consonant:

For example, 나 only has the initial consonant ㄴ on the left, followed by the horizontal vowel ㅏ.
누 only has the initial consonant ㄴon top, followed by the horizontal vowel ㅜ.