This is spotted spurge, a summer annual. It reproduces by seed that germinates in spring and grows and flowers in summer. Its growth is encouraged by bare ground. In the lawn, you can prevent or minimize the problem by over-seeding to cover thinning spots and by mowing your lawn at a setting of 2 to 3 inches to shade out germinating seeds. Low mowing is ineffective because the plant grows low to the ground, avoiding mower blades. Hand pull the weed where you see it, wearing gloves to prevent irritation from the milky sap. In the garden, use mulch to prevent bare patches, where seed can germinate and the weed can take hold. For more information on control of this nuisance weed, visit the following website:

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7445.html