What is similar between birds and reptiles?
They are both a part of the same phylum Which is chordates. They are both vertebrates that lay eggs.
Neither. Penguins are not mammals or reptiles - they are birds. The differences between birds and mammals and reptiles are: Birds have feathers while mammals have fur, hair or skin, and reptiles have scaly skin. Birds lay eggs whilst mammals (with the exception of the monotremes) bear live young. Reptiles can reroduce either way, depending on the species. Birds and reptiles do not feed their young on mothers' milk like all mammals do.
To some degree. Both birds and reptiles reproduce sexually and have offspring that resemble their parents. All birds and most reptiles lay eggs. One key difference is that reptiles are born largely able to fend for themselves while birds are fairly helpless and need their parents' care. The only reptiles that provide parental care are crocodiles and alligators, which help their young hatch and protect them from predators, but do not provide food.