The difference is that the Luer Lock syringe allows a needle to be twisted onto the tip and then locked in place. This provides a secure connection and prevents accidental removal of the needles of the injection of fluids. This also affects the price and Luer Slip syringes tend to be cheaper than Leur Lock syringes because of this feature.
Luer Slip syringes allow a needle to be pushed on to the end of the tip if a needle is required, all our Leur Slip syringes accept needles in this way.