From what I know they both are used for future tense, what's the difference and can they be used interchangeably?
Well there are few things to point out I guess.
1) When the subject of the sentence is 1st person, both of the grammar points express 의지 (Will, dedication). The difference here is that 겠다 has a stronger feeling of will/dedication towards a certain fact compared to 을 것이다.
2) When the subject of the sentence is 2nd or 3rd person, both of the grammar points express 추측 (Guess, assumption). The difference should be that 겠다 shows speaker's subjective deduction/guess while 을 것이다 is more objective deduction. Though this may not be the case always. This is partially why 을 것이다 sounds more sure than 겠다. Like when you say "That looks delicious" you'd say "맛있겠다" coming from you seeing the food and thinking that.
There may be other factors too but I'd say these are the main ones.
[The sources are different. Mostly "외국인을 위한 한국어 문법"]