Gilts are simply British government bonds.
A very wide range of gilts are in issue at any given time, with remaining times till maturity of up to 50 years. There are still some irredeemable gilts (ones that will never mature, but pay a perpetual fixed income stream) in issue, but no more are being issued.
The yields to maturity (also called redemption yields) on gilts are the risk free rates, over various periods of time, for valuation models for British securities and sterling denominated eurobonds.
Most gilts pay dividends (interest payments/coupon payments) twice a year. Many trade both in their original form and as strips. However, some gilts pay a first dividend at either more or less than six months after the issue date, in order to make all remaining payments (including redemption) at six monthly intervals.
Gilt prices are usually quoted per £100 nominal, and dividend payments are also usually published per £100 nominal, however some systems show prices per £1 nominal. The latter can be convenient as the value of a holding is simply the nominal value multiplied by the price.
More information on gilts is available from the Debt Management Office website.