Hmmm well you can find most of those by googling but I suppose I can provide a quick explanation regardless.
Using get/set code simply makes dealing with variables easier. There will be LOTS of times when you want to perform actions every time something changes (think, saving data as it changes??) but it also makes it easier to avoid accidentally modifying data that shouldn't be.
For example... A mesh in a conquer server. You don't want to accidentally be screwing with a character's mesh or no one will be able to see them. Simply do something like...
public uint Mesh{get{return (uint)(Transformation * 10000000 + Face * 10000 + Body)}}
No one can ever assign to mesh cause there isn't 'really' a mesh variable, it's simply pulling from other variables and calculating them into something you can use!
Inheritance is very useful but can be a bit hard to understand when you're starting out. The easiest way to think about it (imo) is to play stupid word games.
All Trucks are Vehicles but not all Vehicles are Trucks. Whenever you can say something like that about one of your classes (All Players are Entities but not all Entities are Players) and you can EVER see using one of the other possibilities (monsters anyone?) then you should be using inheritance/interfaces.
The main difference between inheritance/interfaces.
1: You can only inherit directly from one other class/struct. This means you could do something like...
public class Player:Entity
You could NOT do Public class Player: BaseEntity, SocketClass, EtcClass
2: Interfaces can be thought of as structures for your class. Everything in an interface must still be implemented in each class. This is useful in some situations and annoying in others (I wouldn't want to write an IEntity interface and then have to duplicate ALL my entity variable implementations inside monster AND player class)
Was gonna write examples but I got tired...
as for proxy stuff...
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There's also some great "make a custom source" tutorials in the pserver release section which goes indepth with sockets and such.