Difference bet. TCP/IP and Named pipes

what is the diffrence between TCP/IP and Named Pipes ??? which one is better to connect the database from the clients place???? and also we have server and 8 clients connected to it. All the clients are logging into the main server with the same username and password. Is there any problem with this login to the database access???
is username and password will clash from system to system when accessing the Database??? or ntserver????