Simplex frequencies are those where a single frequency is used for transmit and receive. This provides radio-to-radio contacts in local situations where line of sight paths are available. Range is several miles to tens of miles or more depending on power, antenna, and location.
Repeater (duplex) frequencies are those used to access repeaters located on towers or hill tops, which allows contact with hams out of simplex range.
Some repeaters are also linked (via 1.25M or 23cm) so that a ham accessing one repeater can talk to another ham accessing a linked repeater. Both ARRG and WORC repeater clubs (see below) do linking.
Another form of linking is via the internet, in which case world wide communication is possible on VHF/UHF. Examples are EchoLink and IRLP.