Learn Swedish: Användaren måste godkänna avtalet samt villkoren. - The user must accept the agreement as well as the terms.

Both mean "and", but samt is much more formal and often translates better to "as well as" or "and also". You will mostly see samt in written Swedish, like in this legal or technical sentence about terms and agreements, whereas och is used in everyday speech and writing.

Why isn't there an att before godkänna?

The verb måste (must) is a modal verb, and just like in English, it is followed directly by the infinitive form of the next verb without the infinitive marker att (to). You say "must accept" (måste godkänna), not "must to accept".

Why does användaren end in -en if the base word is användare?

Nouns that end in -are (which often describe a person doing an action, like a user) are en-words. To make them definite (the user), you only add an -n instead of an extra -en, because the word already ends in a vowel. So en användare becomes användaren.

Is villkoren singular or plural here?