There are 3 main types of 40G transceivers: QSFP+ 40G SR4, QSFP+ 40G LR4 and QSFP 40G PSM4.
QSFP+ 40G SR4 connects multi-mode optical fibers with MPO/MTP interface. The center wavelength is 850nm, which can reach no more than 300m.
The QSFP+ 40G LR4 connects two single-mode fibers with LC duplex interface. The center wavelength is 1271 1291 1311 1331nm, which can reach up to 10km.
QSFP+ 40G PSM4 is the same working principle as SR4, but it connects 8 single-mode optical fibers in parallel operation. The center wavelength is 1310nm, and transmission distance is up to 10km.