Cat5 is for 100Mbps networks, Cat6 is for 10Gbps networks
Most cables are UTP (Unshielded Twisted Pairs)
STP (Shielded Twisted Pairs) are fancy (i.e. expensive) cables meant to shield the cables against electronic noise (i.e. Electromagnetic Interference, EMI).
Cat5, Cat 5e, Cat 6, RJ45, UTP, STP
Cat is for categories. Different categories get different numbers.
Cat3: could run at speeds of 16Mhz (cycles per second, i.e. Hz), could handle 10Mbps bandwidth networks
Cat5: runs at 100 Mhz, handles 100Mbps networks
Cat5e: also runs at 100 MHz, handles 100Mbps networks. Designed to take care of issues in Cat5. 24AWG or 23AWG
Cat6 and Cat6a: can handle 10Gbps networks. Cat6 cables usually are heavier and thicker and have a larger conductor. 22AWG wire.
RJ45 is the connector for networking cables. Resembles a telephone connector (RJ11), slightly larger in size.
UTP means Unshielded Twisted Pair. Most cables are UTP.
STP means Shielded Twisted Pair. They have a foil covering the cable, to protect against noise (Electromagnetic Intereference, EMI. mobile phones, computers, radios systems, microwaves, electric motors, things can cause interference). Connectors for these are also shielded, they have a metal piece wrapped around the connector.
What cable it is is usually printed on the cable
More twisted cables (twist per inch) tend to handle faster signals
Number of twists is another thing that differentiates the Cat level of a cable. Higher Cat levels are more twisted (and heavier as a result)
There are 3 dire ratings for networking cables. PVC is basic. Riser has got a good amount of shielding. Plenum is super fire retardant cable.
FYI, you can use a networking cable for a telephone. It’s essentially the same, just more twisted pairs. When wiring your home, you don’t need to run a telephone cable AND a networking cable. One type of cable can handle both purposes.