It really depends on context and usage, at least historically. The prefix could mean decimal (e.g., 1 kilobyte = 1000 bytes) or binary (1 kilobyte = 1024 bytes). You commonly would have seen the former on things like hard drive sizes and network speeds. So for instance, a 1 gigabyte hard drive would really be 1,000,000,000 bytes.

The IEC defined a new set of prefixes to get around the ambiguity. For instance, the kibi- prefix means 2^10, so a kibibyte is 1024 bytes.