very basic applications & explanation, but it would be easier if you told us, what you were planning on doing
I find the easiest to understand examples are: you want to connect to the internet. but it works with any network. it's just an application most people have a basic understanding about and can apply to them.
you got a modem from your ISP. but it doesn't come with enough LAN-ports for you needs (for example modem has 1 lan port, you got 3 computers) --> buy a switch or a hub, the switch will allow you to connect all your computers to the switch and only the switch itself is plugged into the modem and relays your internet connection to all your computers. Of course same applies for a local network when your Router doesn't have enough LAN ports, or in a small office for example, to avoid dragging 15 cables around the office if the router isn't located closely to the computers.
the difference between a hub and a switch is that a switch relays the incoming data only to the machine that requested it while a hub relays it to all machines connected. in theory if we're connected to the same hub, I can see every bit of data that you send and receive over the network. if we're connected to the same switch, I cannot.
a Router's task is to route the data stream. it seperates and connects independent networks for example and manages data streams.
for example, you own a small b&b. and you got an internet connection from your ISP, let's say a symmetrical 50Mbit/s line. you offer your guests internet access, but since your company's computers and your private laptop use that internet connection as well, you want to seperate those networks from one another. Also you want to make sure that your guests/customers can't use more than half the speed.
the thing you do is buying a router and setting up a network for your guests, all guests can only connect to your router but not to the modem directly, which only you can and do (or you get a second router for your private network all along), and limit the network traffic on the router so it can't distribute more than 20 Mbit/s
another application would be if you're supplied with a modem by your ISP but you don't trust your ISP or the brand that modem is, or maybe it's just junk or you got a very good WiFi-router, you connect it to your ISP's modem and connect only to your router. there are some upsides to this: your Router might be of better quality than the modem, if your ISP's modem comes without WiFi but you need WiFi the router can supply it, if you like to set up a personalized SSID & password but your ISP due to poor service resets your modem once a month your WiFi data isn't touched by that (I'm having the exact issue, setting my own WiFi password but once a month the ISP resets my modem and the password is resetted with it)
a Bridge is a bit more complicated due to the variety of tasks it can do. one is for example: let's say you own above mentioned B&B. and you got an IT specialist to service your network. he tells you he can only service your network with this one special router because this is a router where he got the certfication license and possibility to access from the outside. but sadly, your ISP only allows you to use the modem supplied because the modem's MAC address is mandatory to access the ISP's backbone infrastructure. thing you do is, set up the ISP's modem as a bridge which doesn't do much more then telling the ISP's servers it's MAC address while the router of your IT specialist does all the work behind it. This is one basic task you'd set up a bridge. There are many more.
so I hope and didn't make any too big errors there (anyone feel free to correct me)
and if not, I hope it answered some of your questions and uncertainties.
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