GSM extended characters are the few symbols (such as €, [, ], {, }, \, ^, |, ~) that the 7-bit alphabet encodes with an escape, so each one counts as two characters toward the 160 limit.
The GSM-7 basic table has no room for some common symbols. The standard reaches them through an escape character followed by a code, which keeps the message in GSM-7 but costs two positions. The set is small: the euro sign, form feed, square brackets, braces, backslash, caret, pipe and tilde.
They are a trap for a counter that only counts characters: a message with three euro signs is three characters longer than it looks. A text at exactly 160 with one bracket is two segments.
At SMSMeteor: the counter weighs extended characters correctly; the API's segments field is computed the same way.