What is ST******8AS Media?

Question

I have been receiving the spinning color wheel for about a month whenever I use Safari. 
The problem is getting worse and basically everything comes to a standstill. 
Opened Disk Utility and the first item on the left is: 500.11 GB ST3500418AS Media
Clicked on that and then on Verify Disk and received the Error Message:
Storage system verify or repair failed.
I tried reading other questions posted to the Support Community but I am admittedly not a tech person.
Could not find anything that explained what ST3500418AS Media actually is or what it does.
Do I need this? 
From reading some of the posts I checked the Applications in Finder and I only downloaded
Google Earth in December and a Hewlett-Paclard printer update today.
I did move Google Earth to the Trash and that is where it currently sits.
  Model Name:          iMac
  Model Identifier:          iMac12,1
  Processor Name:          Intel Core i5
  Processor Speed:          2.5 GHz
  Number of Processors:          1
  Total Number of Cores:          4
  L2 Cache (per Core):          256 KB
  L3 Cache:          6 MB
  Memory:          4 GB
  Boot ROM Version:          IM121.0047.B1E
  SMC Version (system):          1.71f22
  Serial Number (system):          C0*******HJF
  Hardware UUID:          6B810C41-6580-552B-8789-DEDF89BA1606
Macintosh HD:
  Mount Point :          /  Capacity : 498.93 GB (498,929,332,224 Bytes)
  Format : Mac OS Extended (Journaled, Encrypted)  Available : 476.44 GB (476,444,110,848 Bytes)
  Owners Enabled : Yes  Used : 22.49 GB (22,485,221,376 Bytes)
  Number of Folders : 97,738  Number of Files : 384,703
<Personal Information Edited by Host>

Answer 1

If you receive a disk error message, then Repair the drive.
Repair the Hard Drive and Permissions - Mavericks, Lion/Mountain Lion
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Repair
When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list.  In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive.  If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported then click on the Repair Permissions button. When the process is completed, then quit DU and return to the main menu. Select Restart from the Apple menu.