1) Decoration - to make better in outlook by adding additional matters.
2) Maintenance - usually does not change much in the outlook but to keep the same by fixing what needs to be fix only to remain the same. Just like keeping your body fit by regular exercises and checkups.
Therefore, 1) is about adding to what you already have as a form of improvement in outlook, and 2) is keeping things the way they are and only add when that is missing from before to bring back the old look.
Both "decoration" and "maintenance" have to do with adding something to the house. However, there is a slight difference.
"Decoration" is to add something to the house so that it becomes more beautiful. For example, you have bought a house. It is empty. The wall is white. So you would paint the wall with the colours you like, and buy special furniture and other things to make it nice.
"Maintenance" is to maintain the house in good and useful conditions. For example, after 20 years, the wall of your house becomes dirty. Some paints come off. The pipes leak and have yellow water come out. You would repaint the house and change the pipes for "maintenance".