Legio Lanciarii are spear-armed legionaries, a force with some of the elite status of imperial guards. As well as spears, they also carry javelins, making them a handy force for any Roman commander.
Historically, the Lanciarii units seem to have been light infantry integral to the legions, but, over time, these men were separated into larger detachments. Some then lost their original purpose, but kept the name of "Lanciarii", in much the same way modern armies have kept fusileers, grenadiers, rangers and jagers, even tough they are all infantrymen of one kind or another. The Lanciarii do seem to have had an elite quality to them that made them distinctive elements of the field armies, and also seem to have been a part of the (extended) imperial guard after the dissolution of the Praetorians.