This is the advanced missile-defense system Russia is selling to US allies

Saudi Arabia agreed to purchase Russia's advanced S-400 Triumf missile-defense system on Thursday during King Salman's visit to Moscow.

The agreement marked the second time in the last month that a US ally has agreed to purchase the S-400 system from Moscow.

In September, Turkey finalized and made a deposit on a $2.5 billion deal for the S-400, deepening tensions between Washington and Ankara.

The S-400 is a very capable missile-defense system, somewhat comparable to the US's MIM-104 Patriot, according to CSIS.

Here's what it can do.