The three images above depict Garlic Mustard (Alliaria petiolata), which is found across North America and Europe, as well as parts of Asia and Australia.
First year plants form lumps of small leaves (rosettes) and usually do not flower. In the early spring they are small - just a few cm in diameter with a slightly jagged lilly-pad shape. Later in the summer these get bigger, the edges are more jagged but still have a sort of lilly-pad shape.
Identification of adult plants is easiest if you can find at least one plant that still has flowers.
Flowers are small (6mm or 1/4 inch in diameter), each with four white petals. Leaves on the main stem change from lilly-pad shape near the base of the stem to more spear-like with jagged edges near the flowers.