DoS explanation
Imagine a post office.
It has an entrance, a counter with a clerk who deals with the customers and their packets. The clerk is a multi-tasking talent with a lot of arms to deal with packets on the counter.
The counter has a certain width, so a certain number of customers can be processed at the same time.
The clerk has a small adress book with adresses where to send the packets. Some pages are blank, he can input new addresses.
Behind the counter and the clerk are a certain number of shelfs, where the assistant of the clerk puts packets until they can be further processed and sent.
There are now several cases for a DDOS / DOS attack in this metaphor.