All mammals have various things in common which are; they are warm blooded, they nurse their babies and they bear live young ones. This means that they have placental development and mammary glands in their bodies. They also have hair growing on their bodies.
2 Additional Answers
All mammals feed their young ones with breast milk as well as take care of them. The length of time taken in caring for the young ones typically varies from a number of weeks for mice to several years for humans.
The characteristics that all mammals share is that they are warm blooded and are live bearing, that is their young are born alive, with the exception of the platypus.