The litre is identical to the cubic decimetre (dm³), although it wasn't always so (see history below). The symbol L is preferred to l to avoid confusion with the number 1.
1 dm is 0·1 m or 10 cm, so 1 dm³ is 10×10×10 cm:
This, of course, means that there are 1,000 cm³ in a litre, or that 1 cm³ is equal to 1 mL.