Cells & Ranges
Excel worksheets are often described as looking like "graph paper", which tends to be a workspace that most users are unfamiliar with. The quantitatively inclined tend to find comfort in graph paper, whether as a result of their experience with the format or because we all share at least a slight degree of "OCD".
The main secret to using Excel is to hold in your mind that Excel is an "Object-Oriented" application, in which the computer and application treat everything inside as objects. As a result, each worksheet cell is an object that has its own unique identifier or cell "address". Once we tell Excel which cell object ("A1" or "C14") we want it to work with, the application does the rest.
Even better, is the flexibility Excel provides in that it can treat groups of cells, or "ranges", as more complex objects with the same ease and simplicity. The range "A1:B2" is a group of 4 cells (A1, A2, B1, B2) that Excel will treat as a single object. It enables us to merely highlight a group of cells, and have Excel deal with the entire group.
In the case of the SUM Function, we can command Excel to add up all of the values in a complex range without actually typing in the range addresses simply by highlighting the range we want Excel to evaluate.
For more advanced users, the Name Box can be used to actually name ranges (e.g. "Sales_Q1") in cases where very large or non-continuous ranges are required. Once the range is highlighted, simply type the desired name into the Name Box and hit "Enter". Just remember that the Name Box doesn't allow spaces and dislikes some characters.