What is the difference between sadaqah and fitrah? When we donate some money excluding Zakat money, then what will be the right term to refer that?
Sadaqah ( صدقة ) is the generic term for any charity - wealth that is given as an act of worship, to seek Allah's approval. It is inclusive of both obligatory charity and voluntary charity. And sometimes one may use this term to mean only one of the above. For example the Quran (e.g. 9:60) and hadith (e.g. Muslim 979a) at times use this term to actually mean only Zakat. While when people use the term they usually mean only voluntary charity.
Zakat ( زكاة ) is the name of a specific type of charity which is obligatory. Typically it refers to the charity that is obligatory on a Muslim at an ordained rate (e.g. 2.5%) when he/she owns a minimum threshold of wealth (nisab) for an ordained period of time (one year). For example if you have owned 1000 grams of Gold for a whole year, then you must pay 25 grams from it (or the equivalent price) as charity.
Sadaqah al-Fitr ( صدقة الفطر ) , Zakat al-Fitr ( زكاة الفطر ) and Fitrah ( فطرة ) is the name of another specific type of charity which is obligatory on every Muslim at the end of Ramadan. The amount to be disbursed is fixed per person. So for example if you are a family of 5, you would give 5 saa's (15Kg) of rice after every Ramadan.
Sadaqah al-Tatawwu‘ ( صدقة التطوع ) or Sadaqah al-Nafilah ( صدقة النافلة ) is the name of a specific type of charity which is not part of an obligation, but rather is given voluntarily. For example you might just feel like doing a virtuous deed and decide to give $20 as charity. There is no restriction on the time, amount, preconditions or recipients of this charity.
What is the difference between sadaqah and fitrah?
In most cases Sadaqah means Sadaqah al-Tatawwu‘. Here the difference between them is that one is obligatory while the second is voluntary. One is due at a special occasion and has a prescribed amount, while the other is at your discretion.
Otherwise Sadaqah is used in its wider meaning. Here the difference between them is that Sadaqah is general while Sadaqah al-Fitr is specific. Fitrah is always a type of Sadaqah but not every Sadaqah is Fitrah. As another example of this concept: The term 'book' is general but 'Quran' is specific. The Quran is a book, but not every book is the Quran.
When we donate some money excluding Zakat money, then what will be the right term to refer that?
If you mean optional charity, then you can use either Sadaqah al-Tatawwu‘ or Sadaqah al-Nafilah or even plain Sadaqah.
والفطرة: صدقة الفطر، والخلقة التي خلق عليها المولود في رحم أمه، والدين