What was the problem with the Hubble Space Telescope and how was it solved?

The Hubble telescope was launched in 1990, in the low Earth orbit and remains in operation. It was built by NASA with contributions from European Space Agency. There was a serious problem with the optical system and the analysis showed that the cause is the primary mirror that had been polished to the wrong shape, introducing spherical aberration. It was solved by sending servicing mission in 1993, by installing a set of specialized lenses to correct the flawed main mirror in the telescope. Including this, there have been total five astronaut servicing mission to Hubble.