Which is correct?

Chocolate ice cream is better than vanilla.

Chocolate ice cream is better then vanilla.

The correct choice is the first sentence using than.

Than is a subordinating conjunction. It is used in comparisons, as in the example sentence above in which two flavors of ice cream are being compared.

Then is considered a conjunctive adverb. It is used in reference to time, indicating that one action follows another.

We spent the morning sightseeing; then we stopped for lunch at a local restaurant.

Then and than are often pronounced as if they were the same word, which may explain why they are frequently confused in writing.