Cohortor's gStrings is an application for Android based smart phones. I'm not sure if there is one for other phones, as I've only used it on Android. You download the app straight from the Android market and it will download and install within a few minutes. Once up and running, you'll find that the tuner is chromatic and designed for tuning all kinds of instruments. I tune mostly my acoustic guitars with gStrings, but also use it for mandolin and it will definitely work with any stringed instruments. The interface is easy to use and provides a myriad of options for tuning. There are three basics ways to tune with gStrings. The first is to sound a pitch to tune to, and you can scroll and choose any note you wish on the top of the screen. The next way is to use the "Tune Auto" mode which is what I usually use and allows you to play any of the strings on your instrument and it will track what note it is and show you how far off you are. You can also choose the note you'd like to tune to. All of these are valid and it's nice to have options. I don't think there's a manual for this but you can probably find some extra info about it on Cohortor's website.
There isn't going to be any problems with performance with Cohortor gStrings as it can only run on Android based operating systems like a smart phone or tablet device. Whatever you have that has Android on it will be enough to run the app for sure.
While I honestly wasn't expecting a lot from Cohortor's gStrings since it's a free application for your Android based device, I was pleasantly surprised to find that it's extremely accurate and is up there with the best tuners that I've used. It gives you all the options you need to tune in any way you'd like, whether it's by ear or visually. I use this all the time for my acoustic instruments and since it's on my phone I'm never without a great tuner. If you want a great digital tuner (for FREE) and have an Android based device you should definitely get gStrings!