I agree with ei8th. ㅇ is a consonant but it has two different functions:
As he said, in the first case (the most common) vowels in Korean can not stand by themselves, so the ㅇ becomes a silent noun to enable the vowel sound. 아, 어, 오, 우, and so on.
However, when the ㅇ is the last consonant in a character - 빵, 랑, 강, and so on - it has the sound of 'ng' - bbang, rang, gang and so on.
There is however, something that you need to be careful of. When the ㅇ is at the start of a character, such as 어 but it is preceded by a character which has a final consonant (this final consonant sound is called a 받침) then the 받침 sound moves into where the ㅇ is. Example: 먹어요 (to eat) actually sounds like this 머거요 - the ㄱ sound moves into the place of the ㅇ.