Though the Corporation spends much of the game repelling the Runner's intrusions, traces and tags give the Corporation opportunities to attack the Runner.
Some card abilities initiate a trace on the Runner. Traces are marked by the language "TraceX" on a card, with X equaling the base trace strength of the trace. Traces pit the Corporation's trace strength against the Runner's link strength, both of which are increased by spending credits.
The Corporation acts first during a trace, openly spending any number of credits to increase his trace strength by one point for each credit he spends. There is no limit to the number of credits the Corporation can spend on the trace.
After the Corporation spends his credits, the Runner has the opportunity to openly spend credits to increase his link strength. The Runner's base link strength is equal to the number of links (~) he has in play. The Runner increases his link strength by one point for each credit he spends. There is no limit to the number of credits the Runner can spend on the trace.
After the Runner finishes increasing his link strength, it is compared to the Corporation's trace strength. If the trace strength exceeds the link strength, the trace is successful and any "If successful" effects associated with the trace are resolved.
If the link strength is equal to or greater than the trace strength, then the trace is unsuccessful, and any "If unsuccessful" effects associated with the trace are resolved.
Certain card effects result in a tag being placed on the Runner. As long as the Runner has at least one tag, he is considered to be tagged. While the Runner is tagged, the Corporation may, as an action, spend click and 2 credits to trash one of the Runner's resources. Certain card effects can also trigger off of the Runner being tagged, and it is usually dangerous for the Runner to remain tagged for very long.
While tagged, the Runner may, as an action, spend click and 2 credits to remove the tag, returning it to the token bank. The Runner can repeat this action as many times he likes, provided he has the clicks and credits to pay its cost, and as long as he has a tag to remove.