The text after the colon is an identifier for an "Alternate Data Stream". ADS is used to store meta-information about the file. For example, the Zone identifier stores whether the file was downloaded from the internet.
Some specific info: About URL Security Zones, Zone.Identifier Stream Name
Best is to google around for more information. Eg. this, this and this.
Finally, here's Streams, the Sysinternals tool for ADS.
