TL;DR: 世界 – world, 地球 – Earth
世界 (せかい) – means “world”, as in, the world is ending, the world that we live in, etc.
地球 (ちきゅう) – means “Earth”, as in aliens are invading Earth, the Earth is the third planet, etc. This is really odd as the Japanese naming convention for planets are a bit different, but I think it’s really reasonable. Planets other than Earth are “stars” and end with the kanji 星 (せい).
I… think I won’t go over planet vocabulary over here, but if you’re here for the word “earth” (the ground), the kanji is 土 (つち).