If you have synced your iPad with your computer you can keep cites of your apps in iTunes on your computer.
You can also download any apps from the App Store again that you have previously purchased without paying for them a second time so long as you are signed in with the same Apple ID that you used to purchase them. The only ca are with this is that sometimes apps are removed from the App Store so you wouldn't be able to download those again.
Best suggestion is to connect your iPad to a computer. Preferable the computer you normally sync with. Start iTunes on that computer, select your iPad in iTunes and select summary from the row of tabs presented. You should find a transfer purchases option and clicking on that will allow you to transfer your apps into iTunes for later retrieval.
After that, You can delete any purchased app from your iPad by tapping and holding on the app until your apps jiggle and display an X. Tap the X to delete the app.
all downloads are containted in the app store. so even if you delete them, by going into purchases within app store directly on the tablet will give you the feature of redownloading them once more.