Yellowstone is a Super volcano that has had at least 3 eruptions that measured 8 on the Volcano Explosivity Index . The first was 2.1 million years ago, second was 1.2 million years ago and the third, 640,000 years ago. Here is a pic of a display at the visitor center. The Blue depicts Mt St. Helen’s Ash deposit. The other three were the 3 super volcanic eruptions at Yellowstone. The latest having the largest area of ash deposit.
For a more detailed explanation check out this link
https://www.yellowstonepark.com/things-to-do/yellowstone-supervolcano
There are 4 different hydrothermal features at Yellowstone: hot springs, mudpots, steam vents (fumeroles) and geysers. For more explanation of the differences:
http://www.yellowstonenationalpark.com/geology.htm
Here are example of the geysers, hot springs, fumaroles, and mud pots we visited. There are over 10,000 hydrothermal features throughout the park.
Old Faithful- the most visited. Rangers can predict the eruption within a 20 minute window.
Other smaller eruptions:
Bacteria gives these Hot Springs their colors. 
Here is a bubbling mudpot-
One of the most amazing views was the Grand Prismatic Hot Spring. It is the largest in the united states and third largest in the world. We chose to hike to higher ground to get this bird’s eye view.
There were many Geysers and Hot Springs we did not get to. Here is a view of an chilly early morning drive and the Fumeroles (steam vents) are visible for miles.
We spent a day at Mammoth Hot Springs, this is where we saw the large Elk standing on the terrace. Here are some pics from our day at Mammoth Hot Springs:
Still our favorite pic!