question about overprint colour..

Hi all, can anyone explain to me what is overprint colour? how and when i should applied it to final artwork? i've looked at help section in illustrator but doesn't help me much. and if i don't applied overprint colour, will it affecting much in print results especially when i using online printshop?

overprint is something you should not have to worry about. It only effect pressing stuff instead of printing. Don't think printshop that just makes few low amount of prints worry with this because it is all digital print and not pressed.

Overprint is allowing 2 colors to overlap, this happens only on certain pressing molds. Where you have 2 inkts to make a 3rd color. Like your company uses yellow green and blue. You want to press this in a mayor amount. You want to keep it cheap so you make blue and yellow overlap to make the green instead of using 3 ink's just use 2.

oooooo...... thank you very much ... i see i see..... a lot of this happen in my place... at my hometown, a lot of printhouse (in traditional term) still use this technique.. (now i know that called overprint) so nothing to worry about eh.. if i click overprint preview in ai, then some colour is missing, ( like white transparent 70% on black) it isn't a problem, is it?
ok... i got it now.. thanks again for helping me

if your design will be used and printed in offset printers, avoid overprinting other colors except black.