What is yoniso manasikara and ayoniso manasikara?

As suggested in this answer , it is a good idea to investigate this further. What is wise reflection (yoniso manasikāra) and unwise reflection (ayoniso manasikāra)? Yoniso manasikāra is also translated as wise attention, appropriate attention. Why is yoniso manasikāra important? What is it used for? How does one practise it?