haven't checked my english but i'll try to say it in simple words. i also used it before
JDownloader = requirements any JDK.
Its a great download manager purely programmed from java. though it will eat a bit of your system memory. now for its features
instant premium account log-in = automatically logs in your premium accounts
auto extracts with password settings = for frequent uploaders who puts their passwords in their RAR
link checker = automatically checks for dead links
download limits = settings that will set your download limit (pause downloads has also download limit settings so it's recommended if you want to pause downloads yet not breaking any link for those unresumable links)
automatically downloads just paste any link it will let it's program enter captcha's (best for megaupload)
Jdownloader packs = it helps you compile links into a file, wherein you can share to others. (good for a bunch of links eg. 22parts of a RAR file.)
cons = frequent updates needed
no java = no Jdownloader
sometimes it hangs if not maintained
has some issues with lesser known sites.
did i miss any points?