octet and ia5 strings

A quick (though rather basic I'm afraid :) ) question I wondered if anyone
here knew?

What exactly are the definitions of octet string and IA5 string? (Or
alternatively does anyone
know any URL's where the definitions are quoted?) I keep seeing references
to these in the
RFC's that explain the LDAP standards - but nothing defining them.

Thanks in advance

Simon Robinson
Lucent-SDX