A finca is essentially nothing more than a plot of land, with or without buildings. In Spain, there are several types of fincas. For each finca, the Plan General de Ordenación Urbana (Urban Zoning Plan) specifies the permitted use.
In general, there are two types of plots on which one is allowed to build a house:
Of course there are many more rules to take into account, but these are the two main types.
Please note: there are quite a few fincas for sale online that still need renovation, but the plot isn't always legally owned. The rules and regulations regarding plots, building, and renovation are very different in Spain than in your home country, and we're happy to help you with that.
It's also a good idea to check which banks can provide financing for a finca. For example, most of the banks affiliated with Spaanse Hypotheek do not finance rustic fincas, but they do finance urban fincas.