Assange was wanted for questioning. The original prosecutor dropped the investigation into rape, which was misreported as "charges", by Assange himself (see Jack of Kent link on my site)
This was appealed on behalf of the plaitiff by Claes Borgstorm, her målsägarbiträde (no direct English equivalent - a sort of advisor/counsellor. In Sweden, this is a standard process called "överprövning av åklagarbeslut" and Google does a decentish translation of the government page (link from the 4 corners page)
In 2010, 12% of these appeals resulted in a change of decision. In Assange's case, the next-most senior prosecutor reversed the decision and took over the investigation, which was re-started.
In Sweden, as observed by other posters, the decision to charge comes very late - almost immediately before trial. The High Court Summary gives a clear explanation at the top of page 4 (link all over our site but esp "Resources" and "4 Corners".
I have taken these sources from a website that I admin, and the above links came from our article on the "4 corners" show that recently aired on ABC Australia.
The website has abundant links to primary sources and quotes them where necessary. Feel free to com and have a rummage around. The "Resources" page used to live here on my old blog, and was linked to by Charon QC on his blog page here:
Lawcast 219: Carl Gardner on the Assange asylum issues, under the heading "Assange case: a quick reference of legal issues for journalists (with sources)". That page has now moved to WikiWatch.