The terms porch and deck are used interchangeably. What they refer to is the back, or front, of the house where there is a wooden structure that extends out. Like a patio, but raised. Do you need to build a new porch, or replace an old deck? How do you know?
You know you need to replace your existing porch or deck when it becomes rotted. Hopefully, you check your deck and porch for weak spots on a regular basis, and have it inspected every few years. Once rot starts to set into the wood, it is only a matter of time before your deck or porch needs replacement. It is best to do it sooner, rather than later, as rotted wood can give out without warning.
Do you need to build one? This is mostly a matter of aesthetics. There are very few reasons to have to build a porch or deck. If your back yard is very slanted, downwards , then a deck can give you some flat space outside to put a BBQ and some furniture on. It can also provide a place to play if the hill is too steep. Porches are generally if your house is raised above ground level. Then you will likely have a porch and stairs leading up to it. If you simply want to build a porch, you are going to have to check what level your door is at compared to the ground. Sometimes it is simpler to build a walkway or front patio area.
These are some simple things to consider if you are looking to build or replace your existing deck or porch.