These function are not expert function, they are basic Access functions. The function is the "immediate IF function". There is a good explanation of how it works here:
http://office.microsoft.com/en-us/access-help/iif-function-HA001228853.aspx
All we are doing with the first expression is checking if our water usage is in between 0 and 1000. If it is, return a "1", otherwise return "0".
Return similar calculations for the other fields.
For each record, this will return a "1" under the range that values is found in, and a zero for all the other ranges.
Then, we are just Summing up all those "1"s to get a total count.
If you are very green in Access, I would HIGHLY recommend taking a class and/or getting a good introductory book and work through it. It is very important to understand the concepts of Relational Databases, and Data Normalization. Otherwise, if you try wading too far out into the waters of Access without having a good understanding of these concepts, Access will drive you crazy (trust me, I have been there!). Access is much more complex and not nearly as intuitive as Excel, but very powerful when you know how to use it right.
There is a really good article on Relational Databases and Data Normalization here:
http://www.deeptraining.com/litwin/dbdesign/FundamentalsOfRelationalDatabaseDesign.aspx
As far as learning how to do different types of queries in Access, most introductory Access books should do. I learned by using the Microsoft Step-by-Step Access book.