May have come off harsh, sorry for that... Anyway, I'm no geologist, so if someone knows more please chime in...
This is riverstone commonly purchased from any home center that stocks landscaping supplies... The actual type of stone is possibly sandstone or limestone... I know this is river stone because it is smooth... Stones that wash down a river get basically polished. If you found this away from a garden it is a sign water once flowed where you found it...
Fossils are very common in these types of stones, around my part of the woods I find shells in them... And looking at your photos it appears you have found some as well... The stone appears to be a conglomerate of some type, meaning two types of stones together... The shell fossils are the "U" looking things on it... If you cracked this open it will typically crack along a shell and reveal a pretty nifty fossil...
Hope this helps