Thanks for posting this really interesting topic!
I describe my proofreading as "another pair of eyes" and that I do "light editing". Basically I can read/type someone's document and catch any inconsistencies, spelling errors and basic grammar.
Looking into localization work (US English to UK English) has spurred me on to brush up my English grammar skills which, although not poor, can really do with a bit of sharpening up!
Tlcrainich you are so right about proofreading your own work! My "day" job involves quite an amount of typing. I pull it off and read it in hard copy form and still I seem to miss the odd silly little error! Daft isn't it I can see the errors in other peoples work but not my own no matter how often I read it!!
Avocet