It is anonymous on reddit. There are so few people reporting content though that moderators start to recognise specific reporters just by patterns in their reporting, especially if they leave a message in the "other" option. Combine this with the user messaging modmail about their report and some reporters identify themselves.

There's no identifiable information the mods see (unless the report came from another mod or an admin), but the site admins do know who sent in reports in case of suspected report abuse. People can be really annoying with reports, one guy on /r/askscience got so mad his post got removed that he reported every other post and comment for the past week. Again idk it's the same guy that did that but when you get an angry screeds in the mod messages and every post and comment has a report you can guess what happened.