It may be a fungus. But brown spots can also be due to many other things -- insect pests,dog spotting,etc. Best thing to do is take a sample into a reputable garden center (or cooperative extension office, if there is one nearby) and show them the problem. A visual diagnosis is required for problems like this. If it is a fungus, there are fungicides you can use, but there's usually some underlying issue with how you manage your grass (too wet, too dry, not enough fertilizer, mowing too short, whatever). It's good to figure that out and correct it, then in the future, you probably won't need to treat for disease.