Untouchability refers to social practices with respect to those who are avarna, referring to the ancient 4 occupational classes identified by the varna system as exemplified by the Mānavadharmaśāstras. Anyone who is not kṣatriya, brahmin, vaiśya or śudra is avarna ("outcaste; dalit; untouchable"). Article 17 simply asserts that "Untouchability" is forbidden, leaving it to ordinary interpretation to figure out what that really means.
The Scheduled Castes and the Scheduled Tribes (Prevention of Atrocities) Act 1989. "Scheduled Caste" is invoked in Art. 341 of the constitution, which simply says
(1) The President may with respect to any State or Union territory,
and where it is a State after consultation with the Governor thereof,
by public notification, specify the castes, races or tribes or parts
of or groups within castes, races or tribes which shall for the
purposes of this Constitution be deemed to be Scheduled Castes in
relation to that State or Union territory, as the case may be
(2) Parliament may by law include in or exclude from the list of
Scheduled Castes specified in a notification issued under clause ( 1 )
any caste, race or tribe or part of or group within any caste, race or
tribe, but save as aforesaid a notification issued under the said
clause shall not be varied by any subsequent notification
Leaving aside the further definition of those in a Scheduled Caste, the act lists numerous specific actions which are prohibited, the Offenses of Atrocities in Ch II, for example
(a) puts any inedible or obnoxious substance into the mouth of a
member of a Scheduled Caste or a Scheduled Tribe or forces such member
to drink or eat such inedible or obnoxious substance
You can summarize this as "you cannot commit crimes against a member of a Scheduled Caste".
The membership question is reduced to lists, for example
The Constitution, (Scheduled Castes) Order, 1950. There are about 7 such orders for caste, and as many for tribes. Note that only Hindus can be in Scheduled Castes, and Sikhs are dealt with by listing relevant caste members as Hindus irrespective of their religious profession. It is unclear what the law says about the indigenous Christians e.g. Malankara Nasrani, who seem to be likewise treated as Hindu.