Although futsal and indoor soccer might seem to be the same at first sight, there are a bunch of differences between them which is interesting to run through.  

The first distinctive difference between them is that futsal can be played either indoors or outdoors, while indoor soccer -as its name suggests- is played indoors.

Another difference is that indoor soccer can be played with a regular soccer ball while the futsal ball is smaller and bounces less.

As far as the playing surface is concerned, while futsal is played on hard surfaces, indoor soccer is played on turf.Walls surround indoor soccer pitches.
Futsal courts don't have surrounding walls at all. Futsal is faster and more intense, with continuous shots that demand players’ higher mental and physical energy.  

Time and number of players is another difference between both of them: futsal is played in 2 halves of 20 minutes each, while indoor soccer is played in 2 halves of 25 minutes each.
5 players vs. 5 in futsal and 6 vs. 6 in indoor soccer (although there are 5 vs. 5 versions too). 

FIFA rules futsal worldwide while indoor soccer is not regulated by FIFA. Last year the FIFA Worldwide Futsal Cup was held in Lithuania and Portugal won the cup for the first time.