Tamtheel (likening) are two aspects:
1) Likening Allaah to the creation.
2) Likening the creation to Allaah.
As for #1, then this is like the saying of the Jews who say that after Allaah created the heavens and earth, he ‘rested’ on the seventh day.
As for #2, then this is like the saying of the Christians who attribute characteristics solely belonging to Allaah to ‘Eesaa (Jesus). Examples of this are them saying that Jesus is the first and the last (the alfa and omega), and that he accepts worship of man.
Muslims have followed the nations before them in both of these deviations. As for likening Allaah to the creation, then this is the basis and root cause of the deviations of the Ash’arees, Jahmiyyah, Mu’tazilah etc. in regards to the Names and Attributes of Allaah. And as far as likening the creation to Allaah goes, some sects amongst the Muslims have attributes to the Prophet (saw) characteristics of ‘all-aware’ (urdu; hazier nazir), as well as claiming that he can answer your du’aas.
Tamtheel and Tashbeeh:
Likening Allaah to the creation, is also of two types, Tamtheel and Tashbeeh. Tamtheel is to acknowledge that something is equal (in all aspects) to something else, and Tashbeeh is to acknowledge that something is similar (in some aspects) to something else.
Therefore tamtheel is to make a likeness between two things in all perspectives and tashbeeh is to make a likeness between them in most of their characteristics.
[Taken from Ibn Uthaymeens explanation of 'Al-'Aqeedah Al-Hamawiyyah, Pp. 21-22 & 26]