in vivo and in vitro

Could someone explain the difference between in vitro and in vivo? I didn't understand the answer that was posted. Could someone also explain the question that was asked about in vivo and in vitro for the homework 1? That would be fantastic!

In vivo is Latin for "in a living organism." In vitro is Latin for "in glass," and is typically used to mean "in a culture system (not in an animal)." Sample sentence: "S-type Streptococcus pneumoniae grow into glossy colonies with irregular edges in vitro. The same bacteria are capable of killing mice, when injected in vivo."