Bullets, Bond Ladders, and Barbells are portfolio constructions that are used to limit interest rate risk.
The idea behind a bond ladder is to spread bond maturities in a portfolio over fixed intervals, typically 10 maturities in intervals of 2 years each. A typical ladder might have 10 maturities ranging from 2 to 20 years, with an average maturity of around 10 years. Because of this broad diversification by maturity, a rise in interest rates will not impact the portfolio as negatively as compared to a bullet or barbell portfolio construction. If interest rates rise, the loss on the longer term bonds in the portfolio is offset by the fact that shorter term bonds are maturing soon and the proceeds can be reinvested at higher rates.
A barbell portfolio only has 2 maturities - a very short term and a very long term - say 2 years and 20 years, for an average life around 10 years (actually 11 years here, but we are simplifying things). The longer term bonds give a higher yield but have higher interest rate risk. This risk is offset by the fact that the 2 year bonds will mature soon and the proceeds can be reinvested at higher rates. The big risk here is that long rates rise sharply as compared to short rates (a steepening of the yield curve). In this scenario, the loss on the long term bonds will be much greater than the fact that the short term bond proceeds can be reinvested in 2 years at somewhat higher rates.
A bullet portfolio construction only has a single maturity, typically in an intermediate range of around 10 years. The way that interest rate risk is offset here is that all of the investment is not made at one time - rather, the investment is made in installments at fixed intervals. If market interest rates rise, new investment will be made at higher rates, offsetting any loss on the already purchased bonds.
A balloon is a type of bond issue structure, where most of the bonds mature as a "balloon" at a long term maturity date. It is not a type of bond portfolio construction.