Is there a difference between altitude and flight level?
Yes, altitude is below transition level, flight level is above transition altitude.

Altitude is referencing to actual (local) air pressure, while flight level is against standard pressure 2992 / 1013.

The transition altitude is dependent on the location and are definded so both will not mix up.

When climbing through transition altitude, pilots set their QNH to 2992 - and so no longer need to set it to changes due to local weather.

When decending through transition level they set it back to the local QNH - as they are close enough to ground the real altitude (above ground...) is getting important.