You probably already have a router - for your internet connection.
You don't tend to find routers with more than about four network ports.
Nor do you find routers that provide POE power.
So, to add lots more devices, you simply add a network switch with the desired number of network ports and connect it to one of your router's ports with a patch lead.
If you want to use POE to power your devices (as is typical with IP cameras) then
purchase a POE switch.
NB - make sure that you check the POE 'Power Budget' of your switch - both the per port rating and the overall power that the switch can provide ~ to ensure that it has enough power for each camera, and for all your cameras.
PTZ cameras with motors, heaters, wipers, IR LEDs require much more POE power than say a fixed turret camera (maybe 60 Watts versus 6 Watts).
Most modern switches and routers will provide Gigabit network ports.
You can go faster than a Gigabit network nowadays, but it's typically barely worth it unless you have an internet connection rated higher than a Gigabit.
SFP slots in switches allow you to add, for instance, fast/fibre connections to more switches and devices.
Most home / small businesses will run quite happily with unmanaged switches.
Caveat - there will be exceptions to all of the above
Deeper details from specialist sites & web explainers below:
Takeaway
Ultimately, a completely effective networking environment cannot exist sans routers and switches. Switches connect multiple endpoints to enable resource sharing within a limited network. A router has a more expansive purpose, directing traffic from the public or private internet and between local destinations. Knowing the differences and similarities between these devices allows you to use both optimally and maximize your available network capacity.
From
Spiceworks.com
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How to choose between a managed and unmanaged network switch
As mentioned, managed switches are more expensive than unmanaged switches, as they require software patches, updates and -- often -- a skilled professional for implementation. That said, complex networks that consist of servers, wireless access points, PCs and IoT devices often necessitate the configuration options found on managed switches.
From
techtarget.com
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Those are SFP slots -
see Wikipedia for more than you ever wanted to know about them 
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