2.  BTW FWIW IANAL IRL IYKWIM is easier to type and just as understandable to practitioners.

3.  Loses some of the original so for example a picture that is compressed using jpg will not be as detailed as the original but may be perfectly servicable.  BUT if you need the original back and all you have is the compressed copy then you are out of luck.  OTOH, the compression is significant, sometimes on the level of 95%  Mostly used for images, animation and sound.

4.  Removes some of the data but all of the original data can be recovered.  The level of compression is not as great.  Mandatory for compression of programs and for some data.  ZIP and its competitors