Nationalism is pride in where you are from. Patriotism is Racism.

Patriotism is the love, support & defense of one's country, national loyalty. Nationalism is: devotion & loyalty to one's nation; a desire for national advancement or independence; a policy or doctrine asserting the interests of one's own nation. These definitions are from Dictionary.com

In nazi-occupied countries of Europe, the difference was obvious. The nationalists supported collaborationist governments (Petain, Quisling, Pavelic, ...), while patriots were in the resistance (Moulin, Tito, ...). Patriotism is love of your country, nationalism is hatred of anything perceived as opposed to a rather narrow view of values.

Nationalism usually expresses the importance of unity through cultural background, such as language and heritage. Patriotism usually focuses on the support and love of a nation. The nation's values and beliefs. Unity through language and heritage is irrelevant. A simple example. Many Chinese are nationalists(even though they live in a sosialist government)due to their long heritage and they speak a language that has exsisted in China for thousands of years. Factors that binds them together historically. While Americans are often patriotic. They defend and support their nation to the fullest, however as the majority of the population has an ancestry outside of North America, their main language isn't originated in their country, and the exsistance of many different ethnic groups, they are generally not considered nationalistic because they don't share a common historical background. The word "nationalism" has a negative assumption generally speaking after WWII, at least in the western world. However I personally believe that a certain degree of nationalism is positive as it is a part of forming an identity, and strenghtens bonds and culture within a community.

Nationalism is a form of exclusion, you see it from time to time and it is ugly. I always feel Nationalism is more about we are part of a group, we feel pride in a group, you are not part of our group kind of mentality, and if you look at nationalist movements everywhere, that is what they are about from Scotland to Serbia. Patriotism is a more friendly way of expressing your pride for your roots and heritage. Being a patriot is like saying i am (whatver) and i am proud to be from (wherever) and i feel is a much more positive way of backing your country, whether that be in Sport or in general.