Learn Spanish (Spain): ¿Les estás contando algo? - Are you telling them something?

Because les is the indirect object pronoun meaning 'to them'. The people receiving the information are the indirect object, while the thing being told (algo) is the direct object. If you used los or las (direct object pronouns), it would actually mean 'Are you counting them?', since contar also means 'to count'!

Can I put the les at the end of the sentence instead?

Yes! When you have a continuous tense made of the verb estar plus a gerund ending in -ando or -iendo, you have two choices for the pronoun. You can put it before the conjugated verb (Les estás contando) or attach it to the end of the gerund (Estás contándoles). Just remember to add a written accent on the a if you attach it to keep the natural stress.

I learned that 'to tell' is decir. Why does this sentence use contar?

Both verbs can mean 'to tell', but they are used slightly differently. Contar is used when you are narrating something, like a story, a joke, a secret, or a piece of news. Decir is used more for communicating a fact or stating words. Since the speaker is asking if you are sharing a story or news, contando is a great fit.

Could I just say ¿Les cuentas algo? instead of estás contando?