Tinkelbel - I am sure someone could give you a more complete answer, but in short Design 365 is a way to capture an entire year of your regular life. Some take a photo a day, some do a weekly and some a monthly. I have been trying out one where you take 12 pictures on the 12th day of each month. I just couldn't do the picture a day style.
Project Life is similar, except, it is more focused on capturing you life, including journaling.
The best part, is that you can start at any time and do what you are capable of.
Thanks Meagan, you explained quite good