The overly simplified differences:
Investors give you resources and you get to go do your thing. You get to go run your business, build a product, grow your team or whatever you need to do. Some investors want to be more involved than others, but in general, an investor is, and in many cases should be, a little removed from the day in, day out workings of the business. The investor’s job is to bring in the resources needed to free you to run the business.
Partners should be really, really engaged. They are with you, shoulder to shoulder, working in and on the business. It’s not bad to have a partner. Sometimes a partner can help you with your technology, web design, marketing, operations, legal or whatever their strength may be.
Partners are like serious dating relationships or marriages. They stay with you a long time. That could be a huge blessing or it could be a huge curse. You’ve just got to be really careful. Most of the long term dating or marriage relationships that I respect have gone through some really good and really bad times. This is true for business partnerships, too. It gets complicated but can be wildly successful if done well.
That’s the difference. Obviously, there are many legal differences and reams of documents you could prepare to define those differences, but that’s the basic premise.