In the olden days, there were things called repeaters that simply took a weak digital signal and rebuilt it sending out to the other side.
When you joined up lots of these repeaters together they become a Hub.
What goes into one port comes out of all the ports but rejuvenated.
This is a layer one device. It deals with "bits"

Then came Bridges. Bridges filtered packets based on their MAC address or the physical address. If the address existed on the other side of the bridge, the bridge would have "learned" this and stored the address in its tables and would pass the data through. When you joined up lots of bridges it became a multiport bridge or a Switch. Thus ports only send out traffic to the devices that need them, not those that don't need them - that is good news! These Switches work at layer 2 and the unit of data is a frame.

Routers work at layer 3 and are only interested in networks. They deal with Packets of data. They use IP Addresses which are logical (they can be assigned and they can change - might be different for the same physical adddress).

Layer 3 switches are switches that can work at layer 2 or layer 3 or a combination of the two. The ports must be declared to function as a layer 2 port switchport or not. IP routing needs to be turned on in most cases too.
The main beauty of a layer three switch is that it uses an ASIC (Application Specific Integrated Circuit) a hardware chip and is thus faster than software.

The key to understanding all of this is to know the protocol stack lower layers.
Learn layers 1, 2 and 3.

Hope this helps