Jagged edges

[j_ia]

I'm using PSCS4. While using the pen tool to cut images, i realise that the cut out images appear to have some jagged edges whenever they are cut out instead of being smoothly cut out. Anyone has any idea why is that so? it doesn't occur when i use shapes (Eg. Rectangle Marquee tool) to cut out the shapes, it turns out to be smooth. The only problem occurs with pen tool. Help please!

Thanks so much in advance! :)