Most term sheets are not binding agreements, and they don't obligate the investors to make an investment. But, they're very hard to get out of once you've signed.

Usually investors will insist that certain terms are binding on the company, even if a deal doesn't go through for whatever reason. The binding provisions typically require the company to pay the investors' legal fees, and agree not to negotiate with other investors for a certain time (more on "no-shop provisions" later).