Yes, its the 4th layer (transport) of the 7 layers for the OSI model.
Reading your last post/question, if you're also asking about the TCP/IP model, it's the 3rd layer (transport) of the 4 layers for that model.
Port numbers, in IP, are used by both TCP and UDP. Port numbers all quick "sorting" of received packets, to processes that want them. Some applications have been "assigned" specific port numbers. For example, HTTP has assigned to it port 80. So, when a client wants to contact a HTTP server, its uses destination port of 80 and a source port unique to the process making the request. This allows the receiving host to send any received packets with a destination of port 80 to the processes "listening" for those packets, which if there is one, would normally be a HTTP server process.
When the HTTP server responds, it uses the client's source port as the reply's destination port and it might use port 80 for the reply packet's source port. This allows the original client to forward the port quickly to the process that made the request.
Although many applications have "assigned" ports, applications might use other port numbers. I.e. the port number doesn't control an application, it's just a convenience. For example, you could configure a HTTP server to "listen" on port 8080. Now, either the client needs to know that too, or it would need to send its HTTP request to all possible 65K port numbers.
