Yeast packets are one of several forms of what is collectively known as commercial yeast or baker's yeast. They are strains that are specifically selected and cultivated to be as consistent, predictable, and foolproof as possible. These benefits come at the cost of flavor - though there are ways to coax a lot more flavor out of baker's yeast - particularly longer (multi-day) fermentation.
Sourdough is one of several names that represents a symbiotic pair of wild yeast and lactic acid bacteria. Sourdough may or may not produce a sour flavor in the final product. In most cases, it probably doesn't. Generally speaking, using a sourdough culture is more work and much less forgiving.
To learn about sourdough, you might want do do an internet search. You will find lots of information. Once you've done some basic research, I'm sure people will be more than happy to answer specific questions you might have.