Homonyms: Words that are pronounced and often spelt the same but have different meanings.

In Korean homonyms of course share the same pronunciation, but there are often differences in the length of the vowel sounds. You can check the differences by using the Korean-Korean dictionary. Longer vowel sounds will be marked with a [:] and you can also listen to the recordings. Most Koreans don’t know about these differences and the important ones you are likely to pick up naturally like they do so don’t worry about this.  

눈 [눈ː] (snow) has a longer 우 sound than 눈 (eye)
말 [말ː] (words) had a longer 아 sound than 말 (end of; horse)
수학 [수ː학] (maths) has a longer 우 sound that 수학 (study)