Differences between nitrification and denitrification are:
It is the process of conversion of ammonia to nitrites and to nitrates.
This is an intermediate step of nitrogen cycle in which nitrogen is converted into another usable form by bacteria, such as Nitrosomonas.
It is the process of reducing back into free nitrates atmospheric nitrogen.
This is the last step in which nitrogen is released back into its source, i.e. the atmosphere by Pseudomonas (bacteria).