Firstly, we need to establish what is meant by a DVD. If it is a DVD that has been recorded off the tv by yourself, it won't need decrypted but will still need to be converted in to a format that can play on an iPad. Handbrake will do this for you and is free. Search this forum for more info. Prerecorded DVDs are encrypted and need to be decrypted before converting, by a program such as dvdfab. Further reading on the copyright rules on making digital backup copies of DVDs which you own is advised.
So then after converting, you have a digital movie somewhere on your computer. This needs to be brought in to iTunes (file, add file to library). Once in iTunes, it will be found in the computer's iTunes film library. After connecting your iPad, you will then be able to drag the film on to your iPad.