Each layer adds header during "send" operation. The corresponding layer in the destination (or next hop) reads the contents of that header during "receive" operation. We can see this as a virtual connection between layers of those two hosts.
Here is an image from Wikipedia illustrating this concept: https://en.m.wikipedia.org/wiki/File:OSI-model-Communication.svg
Link layer Error Detection: A method to verify the integrity of frames when they are transmited over the media. The receiving side can verify if the received frame has been damaged on transit. If link layer detects a damaged frame, then it discards the frame and could or not ask for a retransmission. (Ethernet never asks for retransmission, it simply discards).
Transport layer Error Control: The sending side send segments that are divided on multiple packets at network layer and each packet on multiple frames at link level.
Each segment travels the network (divided as frames and packets) and is recomposed only at the receiving side.
Between the sending and receiving side could be a lot of intermediate routers. During that transit there could be problems as:
- One or more frames are discarded.
- One or more packets get lost.
- Packets lose their original order.
- A malfunctioning router modifies the data in a packet
These problems will pass undetected through the routers until they get to the error control of the transport layer on the receiving side.
This error control verifies that there wasn't problems on the underneath layers problems and asks for retransmission.
Summary: As most of the errors happen on the media, not on network layer (routers, for example) there is a little redundancy, however only Transport layer is under the obligation of asking for retransmission.
Best Answer
The way I usually explain it is to consider a concrete example of two browsers on a single computer, connected to the same website and logged in as different users.
As Zac says, the sessions are pretty much in the web browser and server, not anywhere else. The OSI model is to help describe things, not mandate how they're implemented.