720,341,564 people use Gmail all over the world.
The total people using Gmail are 425 million. This is because the popularity of Google. Gmail being a Google product is trusted by people.
They use Gmail, an email website to Gmail chat with their other friends who have gmail. They use IM programs. They use Facebook, myspace and Twitter to communicate with known and unknown people.
If one has a smart phone, which many people do these days, you can use your gmail account as an application on your phone. If you have an Iphone you can link your account to the email app on the phone.
yahoo
There are millions of people holding Gmail ID's. This is because mailing is an integral part of our life now. There are 425 million people that were using Gmail till 2012.
Don't use the HTML view. Use the standard view
They are programs or people who deliver messages to other programs or people. They are Instant messaging programs. Ya can't use gmail with yahoo messager. Ya gotta use gmail with google talk.
I suggest you try gmail. I use it and it is easy to use for people!
Use Gmail notifier
Most the people think that Gmail is better. Gmail supports a number of features like docs, drive inside it. Yahoo and AOL do not use such things.
Yes, you can use gmail on the iPad.
You have to add Gmail app for that use. In that app you can add as many accounts you want. You just have to enter in your credentials.