When Bogle broadly refers to "mutual funds" he's generally referring to high-fee, actively-managed, non-index tracking funds; either mutual funds or exchange traded funds (ETFs).
The most practical difference between ETFs and mutual funds arises when/if you ever want to move your account from one broker to another. ETF shares can generally be transferred from one broker to another while mutual funds generally need to be sold and rebought. There are technical differences but those are largely irrelevant to individual investors.
Bogle and others in his ilk advocate for passively buying a diversified representation of a broad market index, like an S&P Index fund. Bogle was a pioneer of the field offering low cost index funds (both exchange traded funds and mutual funds). Warren Buffett is also an advocate in this area and recently won a 10-year bet with Ted Seides, a fund manager, that Vanguard's S&P Index fund outperform any actively managed fund of Ted's choosing.
The general conclusion after many studies is that the performance of actively managed funds trail index funds when you consider the fees. S&P index funds generally charge an expense ratio of about 0.02% to 0.10% while actively managed funds can be as high as seven or eight times as much. Over the years the compounding effect of the increased fees materially erode your nest-egg.
Whether or not you choose an ETF or a mutual fund as the vehicle, the idea is to invest in a low-cost broad market tracking index fund over an "actively managed," expensive, high-turnover stock-picker's fund.
