Shards are in-Scanner objects that are displayed as small fragments circling the top of a Portal on the Scanner. They usually appear as part of an anomalous event.
The goal is to get them to a target Portal by creating Links for the Shards to travel along. Periodically, a shard on a Portal will travel: if there are any Links to other Portals of high enough level, it will move from its current Portal to a valid linked Portal. Shards tend to travel to new Portals rather than backtracking. If a Shard has not travelled after a period of time, there is a chance it may teleport to a nearby unlinked Portal. Once a Shard has arrived at a target Portal, it is considered captured and will not leave the Portal.
The exact rules and times of travel will differ depending on the anomalous event.
Gallery
Shard target
Shard target