There are two kinds of Ops on IRC. The ones you will see often, @nicknames with the @ character, are ChanOps, or Channel Operators. These are moderators of individual IRC Channels. Most ChanOps will help you with your IRC client (assuming they know that software), answer general IRC-related questions, and will help you with disputes with other users in that channel. Be sure to read the channel rules, if available (look in the channel topic, or try typing !rules in a given channel).
The ones you won't see often are IRCOps, or IRC Operators – these people are staffers on the IRC servers themselves, and mostly deal with technical or administrative issues. They rarely get involved with disputes in channels, unless the behavior is degrading server performance (DDOS, massive flooding, etc). If you see seriously illegal behaviors in IRC channels, you can always report it to an IRCOp. On Undernet, you can usually find IRC Ops in #undernet or #help.
For more on getting assistance on Undernet, please visit Undernet.org help.