What country and what type of service do you have? Colors can mean different things in different regions, and the electrician may or may not have follow standard color schemes.
But regardless, you should be able to figure out what the wires do by tracing them back to the source.
You are correct - only 3 wires are needed for a basic wall outlet (hot, neutral, ground). You can find basic info about coloring schemes here. The important rules (for the US) are: hot=black (and connects to the bronze colored terminal on the outlet), neutral=white, and neutral-green (or green/yellow).
There are several main reasons why there can be more than 3 wires:
- outlets are cascaded, i.e. there is an incoming and an outgoing wire with 3 conductors each, one continuing to the next wall box)
- there can be a switched circuit and an always-on circuit (where one incoming wire is connected to a wall switch as opposed to a direct connection to the circuit breaker)
- there could also be a separate circuit for an appliance like a washer/dryer/fridge/oven - you will find it in the service box based on colors
- there could be a separate circuit connected to a GFI switch
Check for these things by locating the wires, based on colors, in other wall boxes for outlets and switches, or in the service panel.
ps: ok, now that we know you are in Germany, much of what I said does not apply (about colors and GFI, which is not optional and not distributed in Europe). But the basic process for identifying each wire remains the same.