When I open my GMail and download an attachment, from my understanding, I use HTTP. What are real life examples of using SMTP (for emails) and FTP (downloading files)?
If you use a web browser to use GMail, you do use HTTP. You could have an e-mail application as a client to GMail, and it could use SMTP to send, receive, and synchronize with GMail. A lot of people do this so that they automatically have local copies of their e-mail.
Before the advent of the World Wide Web, FTP was a main tool used for file transfers. It is still often used today where you don't run an HTTP server, or you have a host that doesn't run an HTTP browser. For instance, many large companies with large networks use an FTP server to hold different software versions for their network infrastructure devices. These devices can use FTP to download code updates, configurations, etc., but they don't typically run HTTP browser software.
In addition to what Ron said, you would be aware of their secure counterparts. HTTPS is an SSL encrypted version of HTTP. For example, whenever you enter a credit card anywhere on a site it should be via an HTTPs:// url. Gmail and Google both try to default to HTTPS now.
SMTP is simple mail transfer protocol. It is the protocol used to send email (not receive). To keep it straight in your head you can use a trick of the acronym by saying Send-Mail-To-People. When I write code to send automated emails, I send them through an SMTP server.
In addition to what Ron said on FTP. There is also FTPS and SFTP protocols to do so securely. FTPS uses SSL encryption. SFTP uses SSH. Basically a more secure way to transfer files between devices. I often used SFTP to transfer logs and other. SFTP is more of a sub protocol of SSH than it is FTP.