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You may want to use a comma in a series of three or more things. This is the much-debated Oxford comma (or serial comma). Although the Oxford University Press uses it (hence the name), American English writers use it more often than British English writers. Whether or not you should use it is a stylistic choice. For example, both of these examples can be considered correct:
* We can invest our savings in stocks, bonds or real estate.
* We can invest our savings in stocks, bonds, or real estate.