The present simple - I talk, you talk, he/she talks, etc - is used to refer to routine, such as 'I talk to my mum every week' or 'Children always talk during class.'
The present continuous, or present progressive as it's sometimes called, is used to describe things happening now, like 'She's talking on the phone' or 'we're talking about a bloke at work who we can't stand.'
The name belies the function, but the present simple is not used for things that we are doing now - that's the job of the present continuous.
Funnily enough, we can also use the present continuous to talk about the future. For example, 'I'm talking to him tomorrow about my raise' or 'at the conference, he's talking about the latest mobile phone technology.'
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