These are the main episodes in the series.
Arc 1: Blood and Guts
Blood and Guts is the first arc of the series lasting a total of 50 seasons, that ended with "Enough is Enough" . The number of episodes changed from the beginning (around 10-20) to the end (around 30-40), and so has the artwork.
Arc 1 was completed in February 2014. According to Waterclam, the ultimate DVD set will be released during the making of Season 51 with the complete first arc.
Arc 2: Return of the Gore
Return of the Gore is the second arc of the series with a total of 50 seasons, which began after Season 50 ended. In the middle of Arc 1, there was a poll asking what people want in Arc 2, with "something extreme" winning.
This arc ended with "Flames That Fizzle". On September, the complete Arc 2 DVD was released.
Arc 3: Still Alive
Still Alive is the third arc of the series with a possible total of 50 seasons like the previous two arcs and begined after the end of Season 100. It started with "All Cats Go to Heaven". Not too much is known about this arc.
This arc is still being released along with it's episodes.