The back of a television is a row of sockets, most of which go unused, one or two of which matter a great deal.
Knowing which is which saves buying a set with too few of the ones you need, and saves a good deal of confusion when something does not work.
HDMI is the one that matters
Almost everything connects over HDMI. A single cable carries picture and sound in both directions, and it has been the standard connection for well over a decade.
The practical question is how many you need. Count the things that will be permanently plugged in. A streaming stick, a set top box, a games console, a soundbar, a disc player. Each takes one.
Three is workable for most households. Four gives room. Two is tight the moment a soundbar is involved, because the soundbar takes one of them and leaves you with a single input.
Running out means either unplugging things or buying a switch, and a switch adds a point of failure to a setup that otherwise works.
What ARC actually does
ARC stands for Audio Return Channel, and it solves a specific problem.
Normally HDMI sends sound and picture into the television. ARC lets sound travel back out of the television to a soundbar or amplifier over the same cable.
That matters because it means everything can connect to the television, and the television passes sound onward. Without it, a soundbar needs its own separate audio connection, and you end up with sources plugged into the soundbar rather than the set.
ARC also carries control, so the television remote adjusts the soundbar volume rather than needing two remotes.
The thing to know is that ARC is usually only on one HDMI port. It will be labelled, and plugging a soundbar into the wrong port is the single most common reason people conclude ARC does not work.
eARC is the newer version, carrying higher bandwidth audio formats. Useful if you have equipment that supports it, and it needs to be supported at both ends.
Optical, and when it is the answer
Optical audio, sometimes labelled TOSLINK or digital audio out, carries sound over a fibre cable.
It works reliably and handles stereo and basic surround. It will not carry the higher end audio formats and it does not pass remote control the way ARC does.
It remains the fallback where ARC is unavailable at one end or the other, and for a lot of setups it is entirely adequate. There is more on connecting audio in the note on soundbars and built-in TV sound.
USB
USB ports on a television do two things.
They play media from a stick or drive, which is useful for photographs and video files.
They also supply power, which is what most people actually use them for. A streaming stick can run from the television's USB port rather than a mains adapter, which removes a plug and means the stick powers down with the set.
Worth checking how much power the port supplies, since some devices need more than a television port provides.
The aerial and satellite connections
The aerial socket takes a standard coaxial connection for terrestrial broadcast.
Some sets also carry a satellite input, which is a different socket and takes a different feed. A set without one cannot receive satellite directly regardless of what else it does.
There is more on what a set needs to receive broadcast channels in the note on aerials and smart TVs.
Ethernet
A wired network connection, and worth using where it is practical.
Wifi is convenient and variable. A wired connection is consistent, which matters most for higher resolution streaming where a dip in speed shows as a drop in picture quality rather than as buffering.
If the router is anywhere near the television, this is a five minute change that removes a whole category of problem.
The older sockets
Composite, component and SCART appear on some sets and not others, and they exist for legacy equipment.
An older games console, a VCR, a camcorder. If you have something that predates HDMI and you want to keep using it, check the set has the right input before buying, since these are gradually disappearing.
Which HDMI version
HDMI versions determine what bandwidth the connection carries, which matters for higher resolutions and refresh rates.
For ordinary viewing it makes very little difference. For gaming on a current console, where higher refresh rates are the point, it matters more, and the specification is worth checking against what the console can output. There is more on this in the note on refresh rate.
Cables matter here too. An older HDMI cable may not carry the bandwidth a newer connection expects, and the symptom is usually an intermittent picture rather than nothing at all.
Working out what you need
Before buying, list what will be plugged in permanently. Count the HDMI ports required, add one, and check that ARC is present if a soundbar is likely.
Everything else is a bonus rather than a decision point.
Port details are listed against each model at Cello Electronics.