String or Text Functions- this page lists many frequently used string and text functions. Including LEFT, RIGHT, MID, and CONCAT.
Date arithmetic- for example, how to determine the number of days between two dates
Financial functions- this video covers, in-depth, some of the basic financial functions. While it is a long video, it does have a table of contents in the description so you can jump to the parts that you are interested in learning. It covers functions such as PMT, RATE, CUMIPMT, NPER, and FV.
VLOOKUP- finds data in a specified column, based on the information entered
HLOOKUP- finds data in a specified row, based on the information entered
INDEX/ MATCH- an alternative to VLOOKUP or HLOOKUP