The bond pad is where you stick the wire (or bump, or
column, or ball...). An I/O cell may, or may not (library
developer's choice) integrate the pad with the input or
output buffer circuitry. As a rule it's better to bundle
it all up with its active circuitry, protection, keep-outs
and so on so people can just plop down a known good
off-chip connection.
An "io" might be, for example, a tri-state CMOS output
driver with a pad and ESD protection and even a segment
of a standard power bus ring (so everything just butts
together). Or for analog, it could be just a pad or a pad
plus simple ESD clamps. Lot of variation & styles.