Is a porch just a deck with a roof?
It depends. (Notice how many of our answers begin like that?)

A deck is a structure, usually attached to a building, that supports people and furniture in a somewhat open area.  As some comments point out, because deck flooring is exposed to weather, there are generally drainage gaps in the floor.

A porch is generally a deck-like space with a roof, attached to a building. As also stated, porches often have tight floors since little water/snow lands on them to need drainage.

But there are numerous variations on porches, and these have a significant bearing on their construction.

Weight - The foundation of the porch structure needs to be able to support the roof and a possible snow load (depending on your location) and wind shear force (again depending on your location).

Drainage - Unless the porch is fully enclosed with walls and windows (at which point it is approaching an unheated room), you need some drainage to allow rain or snow that comes in form the sides to exit.  This has a bearing on how you structure the edges of the roof and the edges of the deck. You may need gutters, weep holes or other moisture diverters.

Sides - These may be fully open, railed, screened, partially walled or even windowed-in. All of these factors have bearing on the construction details, the weight bearing system and the drainage.

Lighting - You need to consider whether you need additional lighting if a roof is added. Decks generally have only peripheral lighting, but porches often have ceiling lighting as well.

Ventilation - A porch may need additional circulation and fans are often used, not usually found on decks.

Codes - There may be different permits and regulations depending on whether the structure is considered a deck or porch. You need to check with your local building authority.