What is the cause of Error Code 118, and how can I resolve this?

Above is a screenshot of the error I am getting in my Steam application. It says:

"Unable to connect to server. Server may be offline or you may not be connected to the internet."

I have a connection to the internet and it's working properly, I have no clue what might be the cause of this problem but I can visit the site when I use the "cache" option instead as below;

Any idea what might be the fix for this?

Steam's servers are most likely down. This happens sometimes during heavy traffic, but oftentimes it's during a steam sale like the Autumn sale they had in November or the Holiday sale they are having now.

Here's a summary of Valve's advice, taken from the Troubleshooting part of their help page:

1. Check your firewalls: Your firewalls could be disabling portions of Steam's service
2. Disable background applications: These could be interfering with Steam
3. Virus, spyware, malware, etc.: These could be interfering with Steam
4. Check your router: This could be the main source of the problem.

More or less, if the above solutions don't work for you, this is likely a problem on Steam's end, so just take it easy and wait for Steam servers to go back to normal.