I want to root my nexus 7 tab. The first step is unlocking the bootloader. I want to know that unlocking will wipe the os or just data? Can I start the tab after unlocking
It doesn't wipe the OS (the system partition), only the user data. It's a precaution so that an attacker can't install a new OS to get around Android's security protections on your data. See Why does unlocking the bootloader wipe phone? for more details on why.
When you first boot the device after unlocking the bootloader, if you don't flash a new OS right away, it'll go into the first-time setup wizard (where it asks you to sign into or create a Google account).
When unlocking your bootloader it wipes the tablet, so any data you have on it will be erased. So do a backup before doing anything.
Unlocking the boot loader will not wipeout the OS, that is only done if you install another ROM.
Rooting the Nexus 7 is probably the easiest device to do it on. Here are some links that I have used in the past to accomplish that.
The XDA forum: Wug's Nexus Root Toolkit v. 2.1.9
The NexusToolkit download: Nexus Root Toolkit v2.1.9
Read both thoroughly, everything is explained in an easy to understand way.
The XDA forum is a great source of invaluable information.
The Nexus Toolkit is very concise, with an intuitive UI, that runs on your PC. This will unlock the boot loader, install(flash) recovery, install your boot.img(kernel), and install ROM. There is a vast amount of other features, but you'll find them out when using the toolkit.