Spanish guitar is another name for a nylon-string
guitar (strings which used to be made out of sheep intestines, but commonly
called cat-gut). Nylon string guitars are either what we call
"classical" guitars or "flamenco" guitars (there are some
differences even between these two).
Both "Spanish" guitars and acoustic guitars are acoustic instruments, generally made of tone woods, usually consisting of spruce or cedar tops, mahogany or rosewood backs (or often cypress for Flamenco), and many other varieties.
One cannot simply put steel strings on a classical or nylon strings on an acoustic. If you observe closely, the distance between the strings is more when compared to the latter and the neck of the guitar is taller than an acoustic guitar.
Spanish guitar is more to do with finger picking with your right hand and hence more gap between the strings. Styles of playing are flamenco and classical for a Spanish guitar. However, people normally use plectrums while playing an acoustic guitar as it is mainly used for strumming chords.