What are Vanguard's Admiral Shares? I recently heard about them, and I'm not sure how they differ from the normal shares of their mutual funds I have.
Vanguard's Admiral shares are like regular ("investor") shares in their funds, only they charge lower expense ratios. They have higher investment minimums, though. (For instance, the Vanguard Total Stock Market Index Fund has a minimum of $3,000 and an expense ratio of .18% for the Investor Shares class, but a minimum of $10,000 and an expense ratio of .07% for Admiral Shares). If you've bought a bunch of investor shares and now meet the (recently-reduced) minimum for Admiral shares, or if you have some and buy some more investor shares in the future and meet the minimums, you will qualify for a free, no-tax-impact conversion to the Admiral Shares and save yourself some money.
For more information, see the Vanguard article on their recent changes to Admiral Shares minimums.
Vanguard also offers institutional-class shares with even lower expense ratios than that (with a minimum of $5 million, .06% expense ratios on the same fund). A lot of the costs of operating a fund are per-individual, so they don't need to charge you extra fees for putting in more money after a certain point. They'd rather be competitive and offer it at cost.
Vanguard's funds typically have very low expense ratios to begin with. (The investor shares I've been using as an example are advertised as "84% lower than the average expense ratio of funds with similar holdings".) In fact, Vanguard's whole reason for existing is the premise (stated in founder John C Bogle's undergraduate thesis at Princeton) that individuals can generally get better returns by investing in a cheap fund that tracks an index than by investing in mutual funds that try to pick stocks and beat the index and charge you a steep markup. The average real return of the stock market is supposedly something like 4%; even a small-looking percentage like 1% can eat a big portion of that. Over the course of 40 years waiting for retirement, saving 1% on expenses could leave you with something like 50% more money when you've retired.
If you are interested in the lower expense ratios of the Admiral share classes but cannot meet the minimums, note that funds which are available as ETFs can be traded from Vanguard brokerage accounts commission-free and typically charge the same expense ratios as the Admiral shares without any minimums (but you need to trade them as individual shares, and this is less convenient than moving them around in specific dollar amounts).