What are the levels of the devil's activity against God's children?

There are many assessments of demonic activity, but the simplest scheme to describe the demon's work is the triad of temptation, oppression and obsession.  These three kinds of demonic activity go from the least intrusive to the most intrusive and represent differing degrees of loss of freedom for the afflicted person.  Temptation is an attempted dialogue where the person's freedom remains intact, opression is an attack from the outside, which limits but does not take away the person's freedom, and obsession is an attack from the inside, which severely restricts or harasses the individual's freedom.  Possession is different from obsession only by degree; it is the highest degree of obsession, in which the person  loses most or all of his freedom at any given time of attack.

We can liken the kinds of demonic activity to an enemy army that lays siege to a city: temptation is like the enemy standing outside the city gates asking the occupants to come out and talk: oppression is like the enemy bombarding the city from the outside with shells and mortars; obsession is when the enemy breaches the walls and gets inside the city taking it block by block but is not yet in full control of it.  This is sometimes called "partial posession."  Finally, full possession is when the enemy is fully inside the city and has taken near or full control of the body.  Although this state of full demonic control is rare, it is nonetheless real and extremely dangerous to the spiritual and bodily wellbeing of the individual.

By Fr. Thomas Euteneuer "Exorcism and the Church Militant" page33