Ordering the right part

Hi, Im pretty new to this so this may be a dumb question. I am building a simple circuit and need a 2N3904 Transistor. In the Mouser catalog they have 2N3904, 2N3904_D10Z, 2N3904_D27ZS00Z, 2N3904_D28Z, 2N3904_J05Z, 2N3904TA, 2N3904TAR, and the list keeps on going. I assume to just order the simple '2N3904' but am curious as to what the others are. Thanks a lot.

Often it is just a number suffix to identify one manufacturer from another, and is added by the distributer, not the maker. Other times it is a different package or lead style, maybe a dispensing method, as in loose parts, or parts on tape for autoinsertion. In your particular case it appears to simply separate manufacturers.

I always search by generic number, since often one brand of a part may not be in stock, but the same part from a different maker is stocked.

Looking into ordering information page of datasheet might help. In well documented datasheet this section provides the information about each variant.