Battering Rams are the entry level siege units. They are weak against all troop types and for this reason should not be used to attack armies on resource tiles.
Battering Rams have the highest load capacity out of all Tier 1 troops, however they are also the slowest so are not ideal for gathering on tiles far from your city.
Like all siege units they are strong against city defences i.e. wall traps. You should therefore always include siege units when attacking a player city with lots of traps.