Criminal cases are normally categorized in 3 stages, as:
1. Investigation: Here “investigation” is used in its ordinary dictionary sense which includes collection of evidences, ascertainment of facts, search for relevant data , etc.
Normally, Investigation starts just after registration of FIR and ends with filling of police report, but the stage of registration of FIR is not included in investigation stage.
Investigation is conducted by police officer or by any person (other than magistrate). Magistrate can only direct an investigation but cannot intervene in this.
2. Inquiry: Inquiry is a stage where court applies its judicial mind and it involves taking of cognizance of the case as this involves application of judicial mind.
Inquiry is specifically conducted by court/magistrate or on their instance and not by police officer himself.
Inquiry is different from trial as trial presupposes the idea of offence but Inquiry doesn’t presuppose the offence and it may be related to other proceeding such as security proceeding.
The stage of Inquiry ends just before framing of charge.
3. Trial: Though trial is not defined anywhere but the definition of inquiry u/s 2(g) of CrPC that inquiry and trial stages are mutually exclusive and they never overlap each other.
Trial stage starts the moment charge is framed and ends with judgement.
At this stage court takes the evidence, call the witnesses, evaluate and finally decides the case.
All three staged which are mentioned above are not necessarily followed in every case like investigation stage is skipped in some kind of cases. But there cannot be only trial stage as for that cognizance has to be taken and cognizance is taken at the inquiry stage.