IP stands for Internet Protocol. It is an address assigned to every single computer that has access to the internet that lets websites differentiate between different users. Every computer has two IP address', an internal one, and an external one. The external one is what is shown to websites when the computer connects to them. They are also used to connect to computers/websites. Internal IP's are assigned to all computers in a local network to give them each a unique identity in order to allow them to connect to the internet.
A domain name is a URL using letters instead of numbers to connect to a website. It is used in place of an IP address in order to keep anonymity and have easier-to-remember URLs rather than a string of random numbers.
HTML stands for HyperText Markup Language.
It is named the index because that is the page the website defaults to opening when the website is accessed.
The image html tag is <img>.