@JonathanMoore is correct, this is indeed a Brassica species. The brassica family (Brassicaceae) is a quite diverse family with something like 370 genera and 4000 species. They are found world-wide (apart from Antarctica, although Tryggve Gran tried mostly unsuccessfully to grow Sea Kale during the Terra Nova Expedition to the South Pole). You likely walk past several different types of Brassicaceae every day without knowing that they are there as they are common weeds of untended areas and will take advantage of small cracks in footpaths/pavement to grow.
There are a number of species that are common garden weeds throughout the world. They include many varieties of the Brassica rapa species (known for bok choi, Napa cabbage, choi sum and turnips amongst others, and is known for the oil "canola/colza").
In particular there is one variety called field mustard (Brassica rapa var. rapa, AKA common mustard, wild mustard, wild rutabaga, rape mustard) that is very commonly spread among gardens by its abundant seeding.
It has the long narrow leaves and the clusters of bright yellow flowers characteristic of the Brassica rapa group. However, the Brassica species commonly cross-pollinate and produce mixed varieties, so it is quite difficult to pin down exactly which one you have.