There are several differences between TIPS and I Bonds, and Treasury Direct has a chart comparing the two. Unlike I Bonds, TIPS can be bought and sold on the open market. They can also be purchased through any brokerage platform, such as Fidelity, Schwab or Vanguard, but, like I Bonds, they can also be purchased from Treasury Direct. Unlike I Bonds, TIPS can be purchased in retirement accounts like IRAs. For all practical purposes, there are no limits to how much money you can put in TIPS beyond the $10,000 limit a person can buy each year in electronic I Bonds.
Both I Bonds and TIPS are adjusted for inflation using the CPI, although TIPS are adjusted monthly while I Bonds are adjusted semiannually. Unlike I Bonds, you can see the current value of your TIPS whenever the market is open, though I strongly suggest you not look. You already know what your real, inflation-adjusted return will be if you hold to maturity.