You could be forgiven for thinking that there are many different Buddhas because he has so many different titles and names. Some of these are names and some are more like titles. And some, like Bhagava and Tathāgata can be used to refer to Buddhas in the past, but this is always clear from the context.
Names are given in Pali and Sanskrit when they are different.
Blessed One (Bhagava)
This is by far the most common name used in the suttas. It is more of a title than a name. It is the 9th quality of the Buddha and means that he is the only one who has all nine qualities.
(Note: Blessed is pronounced with two syllables, bless-ed)
Tathāgata
This is another very common way of referring to the Buddha. This is the way the Buddha will most often refer to himself. It is a difficult word to translate, but he gives us a good explanation in the Lokavabodha Sutta. (see pronunciation rule)
Gotama/Gautama
This was his family name. Sometimes in verses other people will also be referred to by this name. Usually it is clear from the context.
Angīrasa
This is a name sometimes used in verses
Sidhatta/Siddhartha
This is what we would now a days call the Buddha’s first name. We actually don’t find this name in the suttas. Sometimes you will hear him called Siddhartha Gautama. (see pronunciation rule)
Sākyamuni/Shakyamuni
This means Sage of the Sākyas, the kingdom where the Buddha was born.