Bombs can be constructed to look like anything and can be placed or delivered in any number of ways. The probability of finding a bomb that looks like the stereotypical bomb is almost nonexistent. The only common denominator that exists is that they are all designed and intended to explode.
A bomb threat is generally defined as a verbal threat to detonate an explosive or incendiary device to cause property damage or injuries, whether or not such a device actually exists.
The latter reason is the most frequently encountered, especially in school settings (and particularly when major exams are scheduled or projects due.) Unfortunately, there is often no way to tell which is the motivation of the caller until after a thorough inspection of the building is conducted. This means that there will always need to be a response to the threat by emergency services personnel (police, fire, and medical).