What is the difference between haram (forbidden), la yajuz (not permissible), makruh (disliked), and ithm (sin)? And does one who commits a makruh act incur sin?

Linguistically, Haram means prohibition. Technically, it refers to a command that definitively requires abstention, and its doer is sinful.

The negation of permissibility is synonymous with prohibition (Tahreem), and often the Imams use this term to mean what is forbidden (Muharram).

Makrooh: That which Islamic law requests to be abandoned, but not definitively, and there is no sin in doing it. There is reward for abandoning it when done with the intention of compliance.

The fundamental difference between them is that performing what is forbidden (Muharram) incurs sin, whereas performing what is disliked (Makrooh) does not.

In religious terminology, Makrooh may sometimes mean Haram, or it may differ from it.

Linguistically, Ithhm (sin) means shortcoming. Technically, it refers to a transgression and a burden in disobedience, and it is metaphorically used to describe that which results in sin.