How to make bootable pendrive to install ubuntu os?

You can download copy of ubuntu iso at http://www.ubuntu.com.

For a pen drive installation, The motherboard has to support booting from a USB flash/pen/key drive. Please consult your BIOS manufacture reference for more details.

An image is a copy of a cd/dvd that can be directly used for burning/mounting. Its more like a zip file or you can say a single file representation of a whole cd/dvd. There are some more features. Like - Copying contents of a cd/dvd and burning them won't make the cd/dvd bootable. There are special flags with image file, that causes the cd to be bootable after burning.