Few days ago when i try to open Gmail,few adds are popping up in my mail.I do not know why this is happening but they troubled me a lot,obstructing me when i want to open a mail.
Ex:
This add is on youtube due to an extension called 550 coupons which you can see in the extension list of your browser [see in the below image].
Even you might have observed some advertisements displaying on your Gmail,Facebook,Youtube......and on many other sites you visit.These adds are displaying because of few people who are very intelligent in making softwares which hack the entire web even on main pages of Google,Youtube..etc to get money when you click on those adds.
So how to avoid these adds displaying on sites you visit ?
The answer is simple,these software hackers use extensions to display these adds on websites.These extensions are linked with the browser you use in your PC.
Below image shows two extensions installed in my browser unknowingly.
So if they are enabled in your browser extensions you have to disable them to avoid these adds.And one important thing is do not remove these extensions because they will install again in your browsers extension list.
If you are using Chrome you can see the extensions in settings tab,using this icon in right top corner. 
So with this tutorial you can avoid these advertisements.
