"Unite" has a more formal feeling than "join" and is always used in titles: United States, United Arab Emirates, Manchester United, etc. It can be used in other contexts in more formal writing or speech. "Join" is used more in informal speech and day-to-day contexts. Here are some examples: 1. fasten, link: to join 2 pieces of wood; to join hands 2. meet: he is joining us after work; will you join us for dinner? 3. become part of: she's joined a tennis club; I joined a new course 4. merge with: this stream joins the Ebro; where the wall joins the roof English has a large number of pairs of synonyms where the word derived from Latin has a more formal usage, and the other word (usually derived from English's Germanic roots) is used in more informal contexts. You can still often use the more formal word in informal contexts (and vice versa), it's a stylistic thing. I hope this helps.