Judging by McGrath's links of poachers and coddlers the main difference would be that coddlers prevent actual contact between the water and the egg. The egg is broken into the buttered coddler which is then closed and partially submerged in the simmering water bath.

The poachers, on the other hand, allow contact with the water in the form of steam, as they are open at the top. Many people also poach eggs by breaking them directly into the boiling water.

Does this seem to sum it up?