Unfortunately, the Yahoo finance website does not mention any units for the data listed. Is the volume for each stock written in shares per day or in $ value per day?
For a stock market-listed security, volume typically represents the number of shares traded. The amount of money involved is called turnover or dollar volume.
A further complication to this relates to the rules of each exchange. Only certain types of trades/trade conditions affect the price of the security. Similarly, only certain types of trades/trade conditions affect the volume of the security. On US stock exchanges, the exact rules are governed by the Consolidated Tape Association, for consolidated trade reporting.
For other types of markets, such as options and futures, the volume represents the number of contracts traded. Such contracts have further details associated with them, such as shares-per-contract, and are often traded on margin, so a turnover or dollar volume isn't applicable to them.
Some markets operate on a round-lot basis (eg. 100 shares per round lot) and sometimes volume can be reported on the basis of round lots rather than actual shares. This type of reporting happened more frequently in the past when there were data storage/transmission limitations. Sometimes reported volume excludes odd-lot trades (that is, trades that are a fraction of the round lot). This is dependent upon the exchange/consolidated tape rules.
Lastly, some markets have no actual reportable volume, such as spot forex, because there is no central exchange for them. Some providers give a figure called "tick volume" which represents the number of price changes (or "ticks") that occur in that time period, but tick volume is an indication of re-pricing than actual trading.
(I have answered this question generically, since it's not specific to Yahoo)