文法的に言うと、現在形と現在進行形の違いです。 Grammatically, it's the difference between the present tense and the present progresive tense. I speak Japanese./I do speak Japanese.→日本語を 話します。 I'm speaking in Japanese.→日本語を話しています。/日本語で話しています。 That's the main difference I can think of immediately. 話します would be used as "I'm going to talk about ___. (これから〇〇の事について話します。) as well. But 話しています mostly be only that way of using. be+___ing, the present progressive tense.

話します represents two things. One means "can speak language" For example, 彼は英語を話します means he can speak English fluently. The other means "will speak/explain about" For example, これから環境問題について話します means I'll try to speak/ explain about the environmental issue. 話しています means "being speaking". For example, 彼らはずっと電話で話しています。means they are / have been talking for a long time on the phone.

話します is present tense of future tense 話しています　is like doing sth ，so here it means be speaking