Is there a different fruit very similar to oranges?
There are hundreds of varieties of citrus fruits, in a great variety of colors, sizes (and very occasionally, even different shapes).  There is really no fundamental difference between the various citrus fruits--they all cross breed and hybridize extremely readily.

It is possible if you post a photo that someone might recognize the cultivar, but other than that all that can really be said is that you have a citrus fruit.