Ribu = 1000, e.g. seribu, dua ribu, tiga ribu. Ribuan = thousands, e.g. There are thousands of cats ... Ada ribuan kucing. Hence, not an exact number; more like an expression. Indo uses hundreds, thousands, millions, etc the same as we do in English. They also use puluhan for tens ... we don't ... e.g. I've been to Jakarta tens of time.