Concrete is that pourable mix of cement, water, sand, and gravel that hardens into a super-strong building material. Sidewalks, foundations, and highways are all made of concrete.
Though people use the words cement and concrete as if they were the same, they're not. Concrete has cement in it, but also includes other materials; cement is what binds concrete together. Construction workers hate when kids write their names in concrete before it hardens: once concrete hardens, it's going to be solid for a long time. When used as an adjective, concrete also means solid. If you've got concrete plans for Saturday, then you have a definite plan.