I like to apply a little foundation before concealer, and a little foundation after. Concealer's used for covering specific scars or acne. Foundation's really more for leveling out your skin tone, but it also can be helpful for evening out the concealer so that it looks natural.
If you only have a few blemishes to cover up and the rest of your skin is clear, you could get away with just using concealer. However if you're looking for a fuller coverage and/or your skin is blotchy, use foundation as a base before applying concealer.