A global macro is a macro which can be used in all workbooks that you have created or will create. For example you might like having you company name and details on the footer or header of your workbooks, so you could create a macro that does this for you saving you time.
A local macro is a macro which only will work for a certain workbook. For an example you might have a monthly report workbook that takes information out from other places, so you might have a macro to do this for you so you don’t have to remember all the steps it takes.
Macros were added to Excel many years ago to make a way to have a set of instructions which are automatically applied. In the early version of Excel they used to record the keystrokes used to perform an operation. With the new versions of Excel you can create a macro by writing it instead of recording it. This program language is called Visual Basic Programming. The early versions of Excel use a more dated version of the VBA. VBA is also found in every Microsoft Office programme so when you have learnt it in one MS Product you can apply your knowledge to the others, although there are slight variations.
You could attend a Excel VBA course which will teach this and more in a lot more detail.