Treasury bills is one type of instruments that is traded in the money market.
A treasury bill is typically a bond that is issued by a central bank or by a government that has a maturity of less that one year. The treasury bill is sold without coupon payment. A coupon is a payment that is made every six month. It is paid as a percentage of the nominal price of the bond.Hence the treasury bill is sold at a discount and then the government will pay the face value at maturity.
These treasury bills are issued by governments in order to raise funds from the market. Because these bonds are issued by the government of a country it is therefore considered to be the safest asset and as a result would have the lowest return of all. Because of its safety it is used very often to calculate the bond spread of other bonds. The bond spread is simply the difference in return between the other bond and the return on a three month treasury bill. The greater the bond spread the riskier the bonds.
Treasury bills are also the most liquid and the most traded of all instruments. This is mainly because banks and other non-bank financial companies are required by law to hold them. For banks they may be used as collaterals in repo transactions with the central banks or in the interbank market to obtain funds. Because they are highly liquid these institutions hold them so that they can be readily converted to cash to settle obligations. Otherwise if they have excess funds they invest them in treasury bill in order to get a decent return. Short term insurers also hold a significant amount of treasury bills because they also need to have access to their fund on short notice.
These treasury bills are issued in two ways. They are either issued through auctions or over the counter. Banks and other institutions participate in weekly auctions. They state the price, the discount rate and the maturity that they want and the central banks will allocate the bills starting from the one offering the highest offered price. After they are issued in the primary market they can then be sold in the secondary market where you can buy them. However you can still buy them directly from the central bank.
As an investor you might obtain treasury bills at any institutions that have them in their portfolio and are prepared to sell them or over the counter at the central bank. But I would strongly advise investing in a money market fund if you want to gain exposure to the money market.
However since treasury bills are the safest of all investment it makes no sense investing a lot of money in them. At best you can invest 5 to 10 % in them in case you want to diversify your portfolio and decrease the riskiness of your portfolio. I would myself advise about 10% in a money market mutual fund, 10% in a bond mutual fund and the rest in other instruments such as gold, stocks, and so on.