User experience (abbreviated as UX) is how a person feels when interfacing with a system. The system could be a website, a web application or desktop software and, in modern contexts, is generally denoted by some form of human-computer interaction (HCI).
UX designers also look at sub-systems and processes within a system. For example, they might study the checkout process of an e-commerce website to see whether users find the process of buying products from the website easy and pleasant. They could delve deeper by studying components of the sub-system, such as seeing how efficient and pleasant is the experience of users filling out input fields in a Web form.
User flow analysis
Low fidelity prototyping
Ideation & concept development
Motion & interaction design