Actually the thing is that a country can have only one national anthem. However, there are other very good patriotic songs, but because they also can not be made 'National Anthems', the most popular and meaningful song from among the 'other good patriotic' songs is chosen and labeled as 'National Song'.
All countries have such National Songs ... along with their National Anthems.

I hav no idea

There is a minor difference between a national anthem and a national song. A national anthem is that the anthem reflects the history and the culture of the country and everybody has to pay their respects to it by giving a standing ovation whenever the song is played. The national anthem represents a particular country. A national song is a song written for a country and does not bear that much of a significance as of the national anthem. It is not mandatory to sing the national song. For example, the national song of our country INDIA - Vande Mataram ... & National Anthem Jana Gana Mana.... 2nd e.g. the national anthem of United Kingdom is "God Save The Queen", and its national song is "Rule Britannia".

Source: just copied and pasted from http://in.answers.yahoo.com/question/index?qid=20110703022258AAN9H0T