Desmos has a host of built-in functions. We're not going to enumerate them all here; for one thing, even Desmos doesn't have a list of them all (really). Here's a list of the functions you're might use in the course of solving Digital SAT math problems:

• Trigonometric functions: SIN(), COS(), TAN(); perhaps surprisingly, we've not seen any Digital SAT math problems that would benefit from the use of the trig functions, though it's possible that could change given that a calculator can be used on all the math problems on the test and given the universal availability of Desmos in the testing environment.
• Inverse trigonometric functions: ARCSIN(), ARCCOS(), ARCTAN(); don't worry if you're unfamiliar with these; they would not be needed to solve any Digital SAT math problem, though on occasion, there will be a clever way to employ one of them (scroll down) as an alternative to a less sophisticated approach.
• Statistical functions: MEAN(), MEDIAN(), TOTAL() (yes, there's a function for computing standard deviation, but you should never need it); these functions take as their argument a series of data values or a list containing data values (the list can be a variable that contains a list, such as a column name from a table).
• Exponents/roots: The sqrt and nthroot commands produce functions, albeit not in function notation; see Exponents and Roots here for more information.
• Points and lines: DISTANCE() returns the distance between two points. MIDPOINT() returns the midpoint of a line given the two endpoints; this can be used to find the center of a circle given the endpoints of a diameter. Oddly, this function is secret; it's not documented anywhere on the Desmos site as of this writing.