What Did She Discover?
Rosalind Franklin's most famous piece of evidence is this image: Photo 51. It depicts an X-ray diffraction of DNA. Taken in 1951, the image eventually led to the conclusion that DNA was composed of a double helix. Such a structure consists of two helices (corkscrew structures) running in parallel.
In 1953, James Watson, an American molecular biologist, visited King's College in an effort to collaborate with Wilkins. When he arrived, Wilkins was not in, so he pressed Rosalind Franklin to share their research. She refused Watson, but on his way out, Watson bumped into the returning Wilkins. The two shared a mutual respect and Wilkins allowed Watson to see Photo 51 (unbeknownst to Franklin).
One month later, Watson and his partner Francis Crick began constructing a model of the DNA double helix in their own labs at Cambridge. After another month, they completed their model and proceeded to publish in Nature, one of the top scientific journals in the world. It is widely held that even though they only had the barest details about Franklin's work, it was a sufficient amount to construct the model. Without her data, the discovery would probably have been delayed for some time and potentially have been made by some other scientific team. Wilkins went on to publish more papers with Watson and Crick, but Franklin received only a small footnote in the first Nature paper.