Most of the data viewed or submitted on the Internet can be seen or modified by others. How do you ensure the data being submitted being submitted or view are secured?
The simple way is to make sure the web site you are visiting are secured. When you are visiting a site with URL starting with http://, the site is not secured. The data that you submit on the page may be modified by other on the network and the data that you read on the site may be already been modified it is loaded on your browser.
HTTP means Hypertext Transfer Protocol. It transfers data in plain text and there is no way to protect the content the protocol transfers. Other can easily modify the content being transferred by HTTP protocol on the network.
To ensure the data being read or submitted are not modified on the Internet, you must make sure the URL you are visiting are always started with https://. The data being read from or write to the Internet are protected. No one (or with very less chance) can view or modify the content of the data.
Basically the web site that starts with https:// is protected by SSL protocol. On other words, it is HTTP protocol over TLS or SSL protocol and over TCP protocol and over IP protocol.
If you are interesting on how these protocols work, you can read their RFC papers:
HTTP – www.ietf.org/rfc/rfc2616.txt (readable plain text)
TLS – www.ietf.org/rfc/rfc2246.txt (encrypted and protected)
TCP – www.ietf.org/rfc/rfc0793.txt (binary)
IP – www.ietf.org/rfc/rfc0793.txt (binary)