First came the Hubble Deep Field, a long look into a tiny speck of black sky that revealed almost 3000 previously unknown galaxies:
The image was constructed from a series of observations by the Hubble Space Telescope. It was assembled from 342 separate exposures taken with the Space Telescope’s Wide Field and Planetary Camera 2 over ten consecutive days between December 18 and December 28, 1995.
Then there was the Hubble Deep Field South, taken in the southern hemisphere:
The observations were made over 10 days in September and October 1998. The southern image reavealed about 3000 galaxies and it included a quasar.
And finally the Hubble Ultra Deep Field:
The images were collected from September 24, 2003, through to January 16, 2004. The ultra-deep image looks back about 13 billion years. In August and September 2009, infrared data was collected from a new camera. With the infrared data, astronomers identified some very distant, and very early, galaxies. The image contains about 10,000 galaxies, many of them deeply red-shifted.
This video (hat tip harleyk) helps to explain it all: