Resurrection is possible, but only under extremely limited circumstances:

1. The body has to have been dead for a very short time, such as the spark of the being has not departed yet. A couple of minutes here, normally. But there are magic abilities that can keep the spark hanging around longer. Often this is a torment for the being whose spark is being held back, however.
2. The body has to be healed to where the spark can reattach - if still fatally wounded, the spark can't be put back in the body. Such repair can be, of course, very difficult sometimes. For example, if an empathic healer takes on the wounds of someone who has died, it's possible that they will die before they can heal themselves. Other healing may take too long. Magic herbs can really help here. On the other hand, sometimes the healing involved can be as simple as clearing water from the lungs.
3. There are other magic abilities which can keep a healed yet dead body from decaying and becoming uninhabitable (which will happen normally even if healed as long as the spark does not inhabit it).

So, basically, the magic to heal and reattach the spark have to happen within a couple of minutes of death, or, the body has to be preserved, and the spark prevented from departing until those conditions can be met. So resurrection is an uncommon occurrence, even when magic is available. And when it does occur, it's more or less because the character is, to quote Miracle Max, "Only mostly dead." Really this is just magical resusitation.

Heroquesting for a Spark

If the spark does depart, and the character is all-the-way dead, it can still be retrieved in theory to reattach to a body - but this requires a heroquest to the otherworld where the dead spark resides. Which is rarely conducted due to difficulty in setting it up, the fact that they're rarely successful when they do occur, and that they're always extremely dangerous in numerous ways including, of course, the questers not returning themselves. That said, since the dead are already on the other side, they can attempt to return themselves. This is simply extremely difficult because of the lack of support, and the fact that crossing by death ties the character very strongly to the other side. But it can happen in theory, and has in practice on a very few occasions.