They don't make it easy for a reason, and that is there are a lot of people that try to cheat the system. Some people think living for a hand-out is a good life, so even thou they could work they don't want too. That leaves a shorter supply of money for those that truely need it. Also many people do not have enough work credits built up to even qualify for SSD. You can check that by getting a copy of your SS work record history. It says how many work credits you have. In those cases if your income is low you might qualifty for SSI instead.
Differences are that SSD is your money paided into the system. If you start working again some might have to be paid back. In SSI, all is usually paided back when you pass on from your insurance policies. Also with SSI if you start getting it keep some in the bank, as they sometimes make mistakes and give you too much and then out of the blue request it back. This happened to someone I knew in Florida.
They often do checks to make sure you are still disabled and unable to work. If you start working and earn over a certain amount they will reduce the amount of SSD or SSI you get, or may stop it all together.