What is the difference between using ‘weil’ and ‘denn’ in German?

Both 'weil' and 'denn' mean 'because', however it is important to remember that they affect the structure of sentences differently:

'Denn' introduces a 'main clause', meaning that it can be used without any change to the word order of a sentence. 

eg. Jana ist eine gute Freundin, denn sie ist immer hilfsbereit und vertrauenswuerdig.

Jana is a good friend, because she is always helpful and trustworthy. 

VS

'Weil' introduces a 'subordinate clause', which means that it sends the verb to the end of the sentence.

eg. Jana ist eine gute Freundin, weil sie immer hilfsbereit und vertrauenswuerdig ist.

Jana is a good friend, because she is always helpful and trustworthy.

TIP: Always remember to put a comma before using either 'weil' or 'denn'!