The so-called 5 Gb/s USB has a data rate of 4 Gb/s.
The marketing data rates for Ethernet are true, i.e. 1 Gb/s Ethernet has a 1 Gb/s data rate, but a 1.25 Gb/s encoded bit rate over the cable.
The marketing data rates for the first 2 generations of PCIe, for all 3 generations of SATA, and for USB 3.0 a.k.a. "Gen 1" of later standards, are false, being advertised as larger with 25% (because 8 data bits are encoded into 10 bits sent over the wire, which does not matter for the user).
All these misleading marketing data rates have been introduced by Intel, who did not follow the rules used in vendor-neutral standards, like Ethernet.
So PCIe 1 is 2 Gb/s, PCIe 2 & USB 3.0 are 4 Gb/s and SATA 3 is 4.8 Gb/s.
So USB "5 Gbps" => 500 MB/s (not 625 MB/s), and after accounting for protocols like "USB Attached SCSI Protocol", the maximum speed that one can see for an USB SSD on a "5 Gbps" port is between 400 MB/s and 450 MB/s.
The same applies for a USB Type C with 2 x 5 Gb/s links.
As other posters have already mentioned, USB 3.1 a.k.a. the "Gen 2" of later standards has introduced a more efficient encoding, so its speed is approximately 10 Gb/s.
The "10 Gbps" USB is not twice faster than the "5 Gbps" USB, it is 2.5 times faster, and this is important to know.
The names for the various bit rates vary between authors and standards.
I believe that the least confusing names would be:
Data bit rate = the rate at which the data bits provided by the user are sent
Signaling bit rate = the rate at which bits are sent over the physical communication medium
The 2 rates are not the same because the user data bits are encoded in some way before being sent. The signaling bit rate does not have any importance, except for those who design communication equipment. For the users of some communication equipment, only the data bit rate matters.
The data bit rate is equal to the signaling bit rate multiplied by the ratio between data bits and the corresponding encoded bits.
5 Gb/s corresponds to 625 MB/s, but for a signaling bit rate it is completely useless to convert bits to bytes, because groups of 8 bits on the physical communication medium do not normally correspond to bytes from the data provided by the user. Only for the data bit rate it is meaningful to be converted to a data byte rate.
>Data bit rate = the rate at which the data bits provided by the user are sent
> Signaling bit rate = the rate at which bits are sent over the physical communication medium
There is a third one: in addition to the line coding, there's the message framing (at the logical level) e.g. USB 3 has a signalling rate of 5Gb/s, it has a raw data rate of 4Gb/s, but it has a theoretical effective data rate of around 3.2Gb/s (400MB/s).
My favorite is when Intel demoed the new ivybridge iGPU by having a guy fire up VLC player to play some footage of a racing game while he pretended to control it with a steering wheel controller.
I looked this up and it's actually even worse than than I thought. When called out he claimed it was being control from backstage.
The so-called 5 Gb/s USB has a data rate of 4 Gb/s.
The marketing data rates for Ethernet are true, i.e. 1 Gb/s Ethernet has a 1 Gb/s data rate, but a 1.25 Gb/s encoded bit rate over the cable.
The marketing data rates for the first 2 generations of PCIe, for all 3 generations of SATA, and for USB 3.0 a.k.a. "Gen 1" of later standards, are false, being advertised as larger with 25% (because 8 data bits are encoded into 10 bits sent over the wire, which does not matter for the user).
All these misleading marketing data rates have been introduced by Intel, who did not follow the rules used in vendor-neutral standards, like Ethernet.
So PCIe 1 is 2 Gb/s, PCIe 2 & USB 3.0 are 4 Gb/s and SATA 3 is 4.8 Gb/s.
So USB "5 Gbps" => 500 MB/s (not 625 MB/s), and after accounting for protocols like "USB Attached SCSI Protocol", the maximum speed that one can see for an USB SSD on a "5 Gbps" port is between 400 MB/s and 450 MB/s.
The same applies for a USB Type C with 2 x 5 Gb/s links.
As other posters have already mentioned, USB 3.1 a.k.a. the "Gen 2" of later standards has introduced a more efficient encoding, so its speed is approximately 10 Gb/s.
The "10 Gbps" USB is not twice faster than the "5 Gbps" USB, it is 2.5 times faster, and this is important to know.