Circuit switching networks as X.25 or Frame Relay use a mechanism that opens a virtual channel from transmitter to receiver before the data transmission begins.
If using Frame Relay you want to communicate from London to Paris, your communication device has to send a signal to its next Frame Relay switch indicating that you want to open a channel to Paris, and that switch sends the same signal to the following switch and so on until the signal gets to Paris switch.
Now that the channel is stablished the communication begins. Each frame from London to Paris goes through the designated channel and when the communication ends the switches close the channel.
Packet switching networks as TCP/IP don't open a predefined channel before sending the first frame of communication.
In the same example using London and Paris, your device sends the first frame and the next switch will have to decide what is the best next hop for that frame and every network device will do the same until the frame gets to Paris.
For each frame the switch decides the best next hop so sometimes the frames will go through one path and sometimes through another depending of traffic, congestion, availability, etc.
Packet Switching networks are more flexible than Circuit switching networks because there is not a designated path from start to end.