Difference is a word for things that are not the same. Identical twins have few if any differences in appearance.
If you dye your hair green, everyone is going to notice a huge difference. People often have differences of opinion. No two people are alike, so there are all kinds of differences between individuals. In math, a difference is the remainder left after subtracting one number from another. Chimps and gorillas are both apes, but there are a lot of differences between them. If something doesn't really change an outcome, you can say, "It makes no difference."