/dev/sda represents the first HDD in your system. If you had to hard disk drives in your system, the next one would be called
/dev/sdb and so on. However, when ever you use your computer to access files, to install operating systems, to manipulate and/or store files on your hard disk, you are actually manipulating "Partitions" instead of the entirety of the disk. Partitions are portions of the hard disk i.e.
/dev/sda which have a starting point and ending point and each partition has it's own filesystem which in layman's terms is a table which contains the name of the file and the starting and ending addresses of the bits that comprise the file.
Now partitions have a similar nomenclature as disks but, they have a number at the end of each name. for example, the first partition of the first hard disk of your system might be called
/dev/sda1. Now if you are coming to linux from windows you probably know this process as "formatting a disk". But in linux, to make your disk ready for use, you have first to create a partition table on your disk. to learn how to create a GPT partition table on your disk using gparted, read the guide on this link . After creating the GPT partition table(scheme) - or an MSDOS one if you prefer - you need to create a partition on the disk. Just follow the guide I have linked above for the creating a partition table and follow the steps I have listed below to create a partition, a filesystem and mount the filesystem. Notice what I just said, ONE MOUNTS A FIELSYSTEM - NOT A PARTITION.
1. Creating a Partition
To create a partition, click on the Partition menu of the Gparted window and Choose "New"
after doing so just click on add on the next windows that pops up.
then go to the "edit" menu of the gparted window and click on "Apply All Operations" then follow that with a click on "Apply" on the confirmation windows that pops up next. You now have a partition table, a partition and a filesystem. To mount the filesystem, you can just click on the new "100 GB Volume" addition to the left hand side of your file manager's window.