Piccadilly Circus is the meeting place of many of London's most famous roads. Here beautiful Regent Street (shopping heaven), famous Piccadilly (Fortnum and Mason's, The Ritz, the Royal Academy of Art), and cultural Shaftsbury Avenue (theaters, Chinatown) intersect. In the middle of it all is the famous 1893 statue of Eros, the winged messenger of love, which commemorates Lord Shaftesbury.
The circus was originally created as part of a plan to connect Carlton House, the home of the Prince Regent who became King George IV in 1820, to Regent's Park. When Shaftesbury Avenue was created in 1885, the area became busy with traffic and advertisers saw the potential for advertising; in 1895 London's first illuminated billboards were put up in Piccadilly Circus. For the next century it was London's version of Times Square but now only one building carries billboards. For history buffs, the name Piccadilly dates from the 17th century and comes from piccadill, a type of collar or ruff.
Given that it's in the heart of the West End, it's hard not to end up in Piccadilly Circus at some time. Hordes of buses pass through and underneath is Piccadilly Circus tube station with entrances popping up at all corners of the circus.