Saiga rifles are made by Izhmash in Russia. They are neutered versions of the venerable AK platform. They are sold in this country in a "sporter" configuration with a unique handguard and no pistol grip. They are otherwise very similar to the AK platform they're based on. Many people chose to convert their Saiga rifles to make them more like a traditional AK. There are pros and cons of doing this modification.
Saigas are made in various calibers including 223 Rem, 7.62x39mm, 5.45x39mm, 7.62x51mm (.308 Win), and shotgun calibers .410 and 12 gauge.
I don't understand why people convert Saigas unless they're in 308 or shotgun calibers. Doing a Saiga conversion on a gun in 7.62x39 or 5.45x39 seems like a total waste. For what it costs (in both time and in parts) to make a Saiga look like an AK, you could've bought an AK instead: a gun that is what it is, not trying to be something that its not. If you want to get an AK pattern gun in 7.62x39 or 5.45x39, go for it! get a real AK, a WASR for $400 out the door or a Polish Tantal AK74 for just a little more. The WASR and the Tantal are real AKs, not some fooled around with wanna-be AK.
I totally understand converting AKs in .308, .410, and 12 gauge because you can't get an AK pattern with that evil pistol grip. The conversion itself is rather involved, not just dropping in a few parts. The conversion requires drilling out rivets, grinding out welds, and refinishing the unfinished portion of the underside of the reciver. If you have one of the .308s, .410s, or 12 gauges, go for that conversion! If you've got one of the other calibers, go ahead and get a real AK. No conversion required for the same money or less.