As an nation that consists of lots of islands, it's no wonder that people from different islands (and even areas) had their own regional languages (this link says that there are over 300 regional/native languages in Indonsia). And even if you live in the same island, there are still differences dialects, though I guess this is a normal case for other languages in the world as well.
However, basically speaking Sundanese and Indonesian language are two different set of languages. Even if you understand Indonesian language, but if you hear me speak only in Sundanese, you would probably not have a clue what I'm saying (or maybe at the most you'll understand just a few words). If I hear someone use Javanese language or Makassarean language or Balinese language, I won't understand almost all of the words either.
Here's what it's said in Wikipedia: Sundanese Language.
Sundanese has two different forms: the "normal" form (the rather crude form) and the "polite" form. The polite form is really tough to learn (the vocabulary for the polite form is different than the normal form) and most people use the less polite form (or combining the polite and less polite form). If I talk to someone much older than me and they talk to me in Sundanese, I'd feel safer using Indonesian language rather than use the "normal" form of Sundanese. However, it's fine to use the normal form of Sundanese to speak to people your own age or younger than yourselves.
I must tell you, though, that there are so many lovely, expressive words/phrases in Sundanese that are irreplaceable because there are no equivalents of those words/phrases in Indonesian language, so many times when I speak with my friends or Mom, I use a hodgepodge of Indonesian with Sundanese words in between - simply because those Sundanese words can explain what I'm trying to say better than any other words in Indonesian.
One friend of mine spent her childhood in the USA for a few years during childhood because her Dad continued his education there. That also resulted in the loss of those years learning Sundanese by immersion. Even though she did end up learning some Sundanese at school until Junior High, even until now she gets confused if we use too much Sundanese in our emails. Or it'd take her longer time to understand the Sundanese texts compared to Indonesian.
Oh yeah, one more thing about Sundanese - the Sundanese scale comprises of "da-mi-na-ti-la-da" instead of "do-re-mi-fa-so-la-ti-do". Before you get all confused, let me introduce you to some Sundanese songs then he he he he he...
First song in two versions (the traditional one and the modern one) called "Mojang Priangan". Mojang = girl (if I still remember correctly it usually refers to an unmarried young woman). Priangan = the area that covers Bandung, West Bandung, Subang, Garut, Purwakarta, and Sumedang.
And another song in two versions as well: the traditional one and a more modern one (with Japanese influence and Japanese words). It's called "Es Lilin". Click here to find out a pic of es lilin.