Commonly, a hacker is a person who uses programming skills and technical knowledge to gain unauthorised access to computer systems for malicious or criminal purposes. The programming community, however, prefers to use the term "cracker" for such persons; they reserve "hacker" for any well-respected, highly skilled programmer.
HTTP means HyperText Transfer Protocol. Anything sent via HTTP should be considered as 100% public and non-private as it can be understood, and modified in transit around the World Wide Web.
HTTPS means HyperText Transfer Protocol Secure. HTTPS connections use SSL and information sent via 128-bit SSL cannot be read, or modified in transit. Originally only used for payment transactions on the World Wide Web, all personal, medical, payment and sensitive information should be encrypted. It is good practice to secure everything your visitors send to and from your site, including basic info like their name and email address - when subscribing to a newsletter for instance.