Jailbreak is just to install and addition store called "Cydia" into ur ios, it is the alternative apps store wihich not only for dl apps, also allow u to unlock some features of the phone, install tones of tweaks and allow u to customise ur iphone like android user do. So, as least for me myself, i think that there is no harm / bad to get iphone jailbroken, the only bad things is, jailbroken iphone will void ur warranty (yes and no if u know what to do) and also cannot update ios over the air.

Good thing u can customize ur iphone in terms of appearance, speed, and much more. Also there are many tweaks in cydia to choose from to make your iphone even better. 

The bad.. None as i recall. Just remember to do it the right way, and also remember to restore your iphone to stock firmware in case u want to send the phone for warranty. Easy as that.   

Well I can name a few bad things from jailbreaking

1. You can't get the latest update from Apple if there's no jailbreak for the latest version. Now  already 7.1.1

2. http://www.infosecurity-magazine.com/view/...n-apple-iphones

3. Pirated apps

4. Unstable and conflicting tweaks makes the phone/tablet unstable

5. Those who are not well verse with jailbreaking might open a security hole for hijackers i.e. thru SSH or malware from untrusted repositories

6. Cannot use paypal to pay GSC movies

That just off the top of my head