User analysis is conducted to identify the target audience of your site. The form of presenting information depends on target audience. User analysis helps one to know more about target audience: age, sex and even preferences and goals. User analysis may be held by a survey or web analytics.
A web site usability audit helps to figure out and then solve the main problems of the site. It is what we were talking about earlier. Usability audit is conducted when a site has a low conversion, i.e. there are few operations, and you want to increase the number of calls, registrations, orders. Site audit may also help when there are such problems as inconvenient navigation, long-term search for elements, poor perception of content.
The purpose of split testing is to compare and contrast two or more variants of a web-page. Two pages are activated at once and then it is analyzed which of them is more popular among users. It is easy when you doubt what color to choose for a certain element, what headline is better, etc. At the end you get the report about the effectiveness of both pages.
By usability testing one may figure out and solve the problems that interface has. Such method of testing is the most accurate and objective because it is held with real users chosen from necessary target audience. Users willing to buy your product are invited, and while they use the site both its advantages and disadvantages are found out.