what is difference between frame relay and router?

Hello all,
Can anyone explains simply, what is the difference between FR and router? and why can't we replace every FR with a router?
Thank in advance,
Ahmad,

Frame relay is used to multiplex or switch between the space (channels) to forward the frames from an input channel. Frame relay does not necessary need to know the statistical load or congestion status of the channels.

Router is linked to the gateway and it is more advanced because it has to know the destination IP subnets. It imposes masks and other security measures.

Frame relay is a synchronous HDLC protocol based network. Data is sent in HDLC packets, referred to as "frames". The diagram below of an HDLC frame may be familiar, since without adding specific definitions of how the Address, Control and CRC is used, the diagram is applicable to IBM's SDLC, to X.25, to HDLC, to Frame Relay, as well as other protocols.

Address Control Data CRC Error Correction

The protocol is similar to that of an X.25 network, except all circuits are permanently assigned

Added after 29 seconds:

https://www.dcbnet.com/notes/framerly.html
this is for Frame Relay in detail ..

Added after 2 minutes:

Routher is a device that forwards data packets along networks. A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISP’s network. Routers are located at gateways, the places where two or more networks connect.

Routers use headers and forwarding tables to determine the best path for forwarding the packets, and they use protocols such as ICMP to communicate with each other and configure the best route between any two hosts.

Frame rely is layer 2 technology to send data consisting of layer 2 switches. Router is a device that works on layer 3 and select the best path to send packets.
Pl look at my notes from Behrouz Forouzan

relay is just used for switching purpose!!!it acts as a switch and to switch the data the two devices should be a part of the same network!!!!
on the other hand a router is a smart device which is used to communicate between two or more networks...or subnets...but the r outer needs to be NAT enabled...