http://senseis.xmp.net/?TsumegoConventions
Conventions
The assumptions in life-and-death (tsumego) problems are rarely spelled out.
They include:
board otherwise empty;
escape from the local position is good enough for life
no effects from corners of the board not represented in the diagram;
seki is obviously a solution to a life problem.
and obviously alternating play
Sometimes it is assumed that
ko is a solution;
there are worthwhile non-local ko threats.
Life-and-death statuses
When asked for the best result in an isolated life-and-death situation, this order usually applies, assuming you are the defender. Reverse the lists, if you are the attacker.
life (any kind)
ko (any kind)
death
Life
This is the order of preference for different kinds of life:
unconditional life or escape to the center
seki
Also note, that life by double ko counts as "unconditional life".
Ko
When the best ko is needed, this order should be easy to remember:
approach ko in defender's favor
direct ko, defender takes first
direct ko, attacker takes first
approach ko in attacker's favor
Also note, that the number of internal threats can be important, since it can in effect change the player who gets to take first.
Other kinds of ko (multi-stage ko, eternal life, etc.) do not fall in this list very nicely, so you have to judge their goodness in the context of the given problem. Generally however, a thousand year ko is better for the defender than a direct ko.
Death
In an isolated problem it does not really matter what kind of ko threats are left behind, so all sequences that kill unconditionally are usually considered equal. The only notable exception is death by double ko, which gives an infinite number of ko threats, and thus has to be considered better than "just dead".
Ladders
There is usually no call to read ladders, but logically speaking we should assume all ladders are good. Note that being chased in a ladder doesn't count as escape.
To be quite precise is a little tricky. Something like this: we assume an otherwise empty board that is semi-infinite in any direction with an unmarked corner, but is a large band (to make ladders work); and we add in a second board as direct summand for ko threats to inhabit.