Satan does mean "adversary", and of course Jesus said "get behind me satan" to Peter because the Apostle had argued that Jesus couldn't/wouldn't/shouldn't be murdered. However, the reason it is always translated as "satan" and not as "adversary" is because the original Greek Gospels do not use the Greek word for "adversary" but the literal word "satan" (satanas). "Satanas" is not a Greek word but a transliteration of an Aramaic word, which means "adversary". But, the fact that the Greek text transliterates "satan", and doesn't translate it, shows us that perhaps the word was meant as a "title" or "name". Therefore, I would say that satan is always a "name" of the devil, and is rarely used to just mean "adversary". Yes, it means Jesus is calling Peter "the devil", but He is not adverse to using harsh language! I also believe Jesus said it directly after He called Peter "the Rock" (you are Peter and on this Rock I will build my Church) to show that Peter's title of "the Rock of the Church" was based on his confession of faith, rather than him as an infallible person. Peter calls Jesus the Messiah and he is "the Rock"; he then tries to oppose Jesus and he is "satan". So much for Petrine succession!
Anyway, that's getting off the point. Satan is, I would say, the name of the devil -- and I would even go so far as to caution you when you read something which implies the devil/satan is our own fallen human nature or passions. Lucifer (light-bearer, as mentioned above) was the angel who tried to usurp God's power, and was cast down from Heaven along with his own army of angels. From then on, he is known primarily as "satan" or "the devil", whilst his army of angels are the demons. In essence, the difference between an angel and a demon is that the first is a messenger of God, whilst the second was a messenger of God. Therefore, you cannot believe in angels and not the devil and his demons. Both are real and active. John Ward mentions that if we cannot control our flesh (more of that below) then the demons don't need to do anything -- their job is already done! But just as true is that if we don't believe/know the devil exists, and tries to tempt us, then it's easy for us to succumb. More than easy, it's inevitable.
I mention that the devil tries to tempt us. Jesus was tempted too, and we can see that the devil here appeared directly to him and his attacks took the form of suggestions which seemed very nice on the surface and... well... tempting! Jesus was not contending with His own human sub conscience, but with a separate being. For us, it is similar. Jesus Christ saw His adversary, as did some of the Saints - like Anthony the Great who physically fought with demons - but the rest of us do not get to see our enemy in such a direct way (thankfully - I'd have a hert-attack). But the form of assault is the same: through suggestions... thoughts that "pop into our heads", without us inviting them or seeking them.
So this is the enemy, and his forms of attack. In fact there is something I might post later which goes into more detail about this, and how these thoughts can turn into sins. What I want to talk about now is the flesh.
The flesh is not our enemy. As much as I want to make clear the reality of the devil, I want to make clear that the flesh is not our enemy. All the talk about conquering and subduing the flesh is about controlling it, and healing it; not destroying it. We are told to flee the devil, but how can we flee our own bodies? All we can do is control them. In other words, we look after (or let God look after) our body's needs, not our body's wants. In other words, the need for food doesn't become gluttony; the need for sleep doesn't become laziness, and so on. The devil plays upon these weaknesses within us - the tendency for our body to control us, rather than us control our bodies, which is why it's useful to know how he does it.