What is the difference in blanching and parboiling?
Both involve boiling water, but there are a number of differences:


blanching has two meanings -- it's mainly used when talking about setting (or enhancing) the color of vegetables, with minimal cooking (only the outermost layer is cooked).  As such, it's typically only a few seconds to a minute dip in already boiling water, followed by a shock (dip in ice water) to halt any further cooking.  It's often used for vegetables that are going to be eaten raw.
parboiling means that you cook something in boiling water to give it a head start.  (Parcooking in boiling water)  Typically, the purpose is to cook an item to speed up the cooking time for some following cooking method.  (eg, partially cook some items in a casserole so that all items will be done at the same time after baking).
And then we have the overlap case -- when you cook something in water to change the characteristics (other than color) of an item before some other cooking step.  For example, we might be trying to extract bitter compounds, or soften a food such that some other processing step can be performed (eg, soften cabbage leaves so they can be used as a wrapper).  In this case, you're typically cooking the item more than just superficially, and the pre-cooking results in a different result than you'd simply get by increasing the time of the final cooking  (eg, the oil blanch for pommes frites, softening the skin to peel a tomato)


So, to help make a decision on which term to use:


if the goal of the step is color change of the ingredient : blanch
if you cook it for only a few seconds, or up to a minute and shock in cold water : blanch
if there's no additional cooking done after this step : blanch
if you could skip this step with no change in other cooking times : blanch
if you could skip this step by increasing the cooking time at some later step : par-boil


... for other cases, where the step can't be omitted without causing problems in the recipe (eg, cabage breaks because it wasn't softened), or final result (too bitter) ... you can often use either term.  You might consider how far 'cooked' the item is after the step if you want to prefer one or the other.  (under about 25% cooked, go with 'blanch', if over 50%, go with par-boil)