Translation:She downloads something from the Internet.
As far as I know, they are pretty much synonymous but in the case of ladda ner the verb is ladda ner, not ladda. It is a part of the verb and with particle/phrasal verbs, you have to use the correct particle. As far as I know, there is always only one choice.
For example, you would never say mata i or mata inne, it's always mata in.
Nor would you ever say ladda uppe - only ladda upp.
You're right about the other ones, but for ladda ner, both ladda ner and ladda ned work. It's the same for all particle verbs with ner/ned, both work. The difference between ner and ned is perceived to be only between more colloquial (ner) and more formal (ned).
But with other particles, it's true that only one version works.
Those two mean 'She has downloaded something from the internet' and 'She is downloading something from the internet', respectively. As others have mentioned, it doesn't matter whether you choose ner or ned. In compound forms though, such as nedladdning and nedladdad, only ned is used.