The Jolly Roger is a pirates' skull with a white helmet with horns, and a yellow mustache shaped like a "W". The symbol has a dim purple circle around it, and "X" crossbones going through.
The World Pirates are likely a formidable crew, as their captain was a prisoner in Level 6 of Impel Down. In the past, they were capable of causing the World Government to fear their strength that they sent the Cipher Pols to deal with them, which is a testament to their strength. Their captain World was strong enough to have the government call upon all the Shichibukai to fight them.
However, despite their strengths, the main members of the crew were still defeated by Luffy and Hancock alone with both of them only having moderate difficulty defeating them.
The crew's ship, Groseade, is a massive ship that takes the form of an island. It was built by the World Pirates during World's imprisonment. It was eventually destroyed by the Marines.
Prior to World's arrest, he treated his crew with great regard, holding camaraderie to the a high degree, following their captain's desire to sail freely. They built the Groseade originally to avoid unnecessary conflict. This all changed when many members were deceived by the World Government to turn on their captain, and the rest were forced to abandon him, allowing him to be arrested. He came to see his crew as unfaithful and disloyal, desiring to destroy the world in revenge, and treated his crew and the ship as dispensable tools.
The World Pirates were formed by Byrnndi World and his brother, Byojack, with the intention of freedom and having adventures. They clashed with many pirates and Marines, and became a formidable threat to the Government. The crew eventually had a clash with Vice Admirals Garp and Sengoku as well as their respective fleets, the World Pirates lower subordinates however were deceived by Cipher Pol to turn on their Captain by shooting him with pistols. Byrnndi World was shocked, and feeling betrayed then collapsed, all the while overhearing his main officers, including his brother, escape. This led him to believe that his crew had abandoned him. World was taken to Impel Down while the rest of the crew went into hiding.
After World escaped from Impel Down, he reunited with his crew, and they destroyed a World Noble's ship. They traveled to Amazon Lily, and, after capturing Boa Marigold and Boa Sandersonia, fought Monkey D. Luffy and Boa Hancock. After the crew was defeated by the two, World told his brother that he used him to carry out his objectives and for nothing more. After World was defeated by Luffy, he apologized to Byojack for the treatment of his crew. World died, and, as the Marines attacked and destroyed Groseade, Byojack stood by his brother. The rest of the crew likely died in the ship's explosion.