To preface this I've been reading Plato's complete works. I'll add a bit but mostly straight from Socrates.

When your thirsty you want to drink. Thirst is desire just like when your hungry you want to eat. If you look at toddler's and as parents we childproof our house because the child will drink anything like bleach or ammonia. Why because they don't know what is good to drink and eat. In time they gain knowledge of what is good to drink and eat. As they grow they gain knowledge to temper their desire.

All desires are like this.

The same principle applies though knowledge tempers desire. Desire is to get the good, things you want and need. Getting what you want with out getting hurt while getting the good.

If your not sure about how to get the good ? Well I know a guy his name is Jesus.