I'm betting you have 2 physical drives in your computer, not 2 partitions.
First, there isn't much purpose to partitioning a drive anymore. You don't need to do it because of size limitations, or performance.
Second, many gaming systems ship with 2 physical drives.
Your primary drive (C:\) is a SSD, not a traditional hard drive. SSDs are a lot faster when it comes to reading data than a hard drive, making them ideal to use with your operating system (Windows) your applications, and games. The downside of a SSD is that it is much more expensive than a traditional hard drive.
The secondary drive (E:\) is a regular hard drive. Windows reserves D:\ for the CD/DVD drive, even if your computer doesn't have one. This hard drive is intended for storing data - your documents, music, photos, videos, downloaded files, etc. Basically, files that you don't access very often, and you need space vs. performance.
You can use the Steam client to move your games from C: to E: but you will notice longer load times. You can also use the Steam client to delete games. Don't worry - you can always redownload a game and deleting games does not delete your save file or progress.
Meanwhile you should also configure your browsers to use E: for downloaded files, and not C:. You may also want to think about moving your user directory over to E: as well.
Going forward, when you install an application, if it's an application you aren't going to be using very often, consider installing it on E: and not C: in order to save space there.