Simply it's that T1's don't produce enough insulin whilst T2's are insulin resistant but they still generally produce their own insulin. This is a simplistic answer.
Type 1 diabetes was formerly called "juvenile diabetes" or "insulin-dependent diabetes," because 70 percent of diagnoses occur before a person reaches the age of 30. However, it can be diagnosed at any age. Only 5 percent to 10 percent of those diagnosed with diabetes have this type. With type 1 diabetes, the pancreas produces little to no insulin.
Type 2 previously called "adult onset diabetes." But according to the Centers for Disease Control and Prevention, type 2 diabetes has been reported among U.S. children and adolescents with increasing frequency over the past 20 years. Ninety percent of those with diabetes have type 2 diabetes. And approximately 80 percent of people with type 2 diabetes are overweight. With type 2 diabetes, your body either resists the effects of insulin or doesn't produce enough insulin to maintain a normal blood sugar level.
I agree with the other posters. Bear in mind that in practice there is a complete continuum from one type to the other and you can sometimes have both. Isn't life complicated.