This would get into a VERY long post so I will give the readers digest version
Sump is nothing more then a place to increase water capacity of your tank and a place for filtration, pumps, heaters, skimmers,
A HOB over flow if for tanks that are not drilled. It is a means for delivering the water from the main tank to the sump. If you don’t have a sump you would not have an overflow.
A frag tank is something you would only have if you frag a lot of your corals and sell or trade them with friends. There is really no reason to have one unless the above reasons apply to you. If so then all it is is another tank used for nothing but housing the frags. It can be plumbed into the main tank to make one system or it can be stand alone.
A fuge is a way of natural exportation of excess nutrients. You can grow macro algae and other plant life which will feed off of the nutrients in your water from time to time you remove the macro as it grows. It is also a great refuge for beneficial things like copepods and mysid.
For more info simply go to Google and type in
What is a (insert name here)
You will find 100s of very very detailed sources
I my case I have all of the above except for the HOB over flow because my tanks are drilled.
If you want to get off to a good start IMO you need the following items
reef ready drilled tank <-- no drilled is fine but if you can aford it get a drilled one you will be VERY glad you did
sump
skimmer
live rock
and a good quality salt IE: tropic marine pro
quality lighting (various types depending on what sort of coral you want to keep)
Before buying any of this I highly advise you to read read and read again in order to determine exactly what type of reef you plan to keep.
this hobby can be very enjoyable and not nearly as expensive as many will lead you to think but it can also be the exact opposite of those things if you do not do your research first.
gl to ya and feel free to ask any questions you come up with.
