The difference between pleading guilty and not guilty. Well if you plea guilty...The prosecution would read out the charge, you would indicate your plea of guilty when asked. You would then add any mitigating circumstances to explain why should be treated leniently. (first offence etc). The trial will be over there and then and no evidence would be required. The sentence would be passed and you all go home. The sentence may be more lenient as your plea was guilty.
Not Guilty Plea. You would lose any guilty plea credit, if found guilty and sentenced. You should then be given all the evidence the prosecution intends to use against you so you can prepare your defence. You will possibly be called back to court for a pre trial review. (Your solicitor will attend on your behalf. If you are unrepresented you will attend in person) It is an admin meeting to decide how long to allocate for the actual trial, based on number of witnesses. Also a time to describe any facilities you may need at the court ie. Video player, tape recorder etc. The magistrates will not be present, just the prosecutor and clerk.
They will set a mutually convenient date and time for the trial where the prosecution will present their case, then you will present yours.
At the trial and on the first hearing also, tyhere will be a clerk present and 3 magistrates. The clerk will chat with the magistrates from time to time as they are fully versant in law. The magistrates are there to judge.
I have limited experience but I hope this is of use.