The difference between muscular strength and muscular endurance is a matter of what is measured for each. Muscular strength is measured by how much work a certain muscle (or set of muscles) is able to accomplish. Muscular endurance is measured by how much work a certain muscle (or set of muscles) is able to accomplish over a set length of time.
Muscular strength is how strong you are. Muscular endurance is how long, for example, you can hold something heavy up. Hence why it's called "endure -ence". Basically, how long you can endure something.
