The first field is the IP address, the second the canonical host name, and any remaining fields are just aliases that will also resolve to the IP address in the first field.
::1
is, indeed, an IPv6 address, which is short for
0000:0000:0000:0000:0000:0000:0000:0001
(the ::
stands in for as many zeros as possible while making the result a valid IPv6 address).