The Tribulation vs. The Great Tribulation

 Are the tribulation and the great tribulation the same thing?

Simple answer: 

Let's dive in.

Dan 9 shows that the tribulation is 7 years, but the great tribulation is the second half of the tribulation. The tribulation is the full 7 years, but the tribulation is divided in two halves. The first half is really bad because of the seal judgements that Jesus enacts, but the trumpet and bowl judgements are going to be really, really bad.

The term "great tribulation" comes from Jesus Himself, explaining the period after the Antichrist sets up the abomination that causes desolation:

15Therefore when you see the ABOMINATION OF DESOLATION which was spoken of through Daniel the prophet, standing in the holy place— let the reader understand— 16then those who are in Judea must flee to the mountains. 17Whoever is on the housetop must not go down to get things out of his house. 18And whoever is in the field must not turn back to get his cloak. 19But woe to those women who are pregnant, and to those who are nursing babies in those days! 20Moreover, pray that when you flee, it will not be in the winter, or on a Sabbath. 21For then there will be a great tribulation, such as has not occurred since the beginning of the world until now, nor ever will again. 22And if those days had not been cut short, no life would have been saved; but for the sake of the elect those days will be cut short.

The confusion comes from misunderstanding the timing of the rapture, and who this portion of scripture is written for. This makes complete sense if you interpret this passage literally and understand that the church will not be here during the tribulation. This portion of scripture is written for the Jews. The great tragedy of the tribulation is that 2/3 of the Jewish people will be wiped out. 1/3 of the Jews will realize that Jesus was the messiah and follow Him. So basically, Jesus is telling the Jews that the last half of the tribulation, after the Antichrist sets up the abomination, things are going to get so bad that He called it the great tribulation, and that it's going to be worse than any period ever experienced in history. That is why it's the great tribulation.