There are some very silly and unhelpful answers here, and one or two sensible ones. I did my training as a police officer in England, so I can only answer for England and Wales.
If a person is arrested they must be told that they are under arrest and why they are under arrest, unless it is impractical to do so, for example the person being arrested is too drunk to understand what is being said to them. In such a case the person must be told as soon as it is possible. Normally a person being arrested will be cautioned at the time and then anything they say after that caution, including any admissions, can be tendered in evidence in later proceedings. If after arrest a suspicion is formed that the arrested person has commited other offences, they must be told that they are also under arrest for those matters.
Don't forget that you can be arrested for any indictable offence, and not just by police officers, and the person making the arrest may use reasonable force to make sure the arrest is successful.
Arrest is totally different from being charged. In all but very minor cases nowadays the Crown Prosecution Service must look at the evidence before it will aprove a person being charged with an offence, and this is why more people are released on police bail nowadays than used to be the case, in order to allow time for a charging decision to be made.
Once a person has been charged, then a court appearance date will be set for the case to have its first hearing.