U-100 and U-40 insulins are like 325mg and 500mg aspirins: the tablets may look the exact same size, but one is stronger than the other. In this case, a U-40 insulin like Vetsulin has enough water added so that there are 40 units in a CC, while a U-100 insulin like Glargine has enough water added so that there are 100 units to a CC.
The difference between U-40 and U-100 syringes are where the lines are drawn on the syringe. For example, a half-CC syringe would hold 20 units of Vetsulin or 50 units of Glargine. Because of this difference, you're technically only supposed to give U-40 insulins with U-40 syringes and only give U-100 insulins with U-100 syringes. But, if you properly convert the number of units you're supposed to give, then you'll find that using U-100 syringes with a U-40 insulin means that the syringes are cheaper, have more variety, are less painful for your cat and are easier to find, and that you can draw up smaller doses more accurately. Which is why a lot of folks here on U-40 insulins use U-100 syringes.
To give a U-40 insulin in a U-100 syringe, multiply the amount of insulin by 2.5. So, to give two units, you would draw to the 5 unit line; to give 0.8 units, draw to the 2 unit line. Vicky and Gandalf have already posted the link to the conversion chart, but I thought this might explain why things are a little better.