Globular clusters are huge, spherical concentrations of stars, tens to hundreds of light years in diameter. The stars they contain are generally much older than those in open clusters. In a globular cluster the density of stars is high, about one star per cubic light year, sufficient to resist disruption by the tidal forces exerted by the Galaxy. Globulars are among the oldest known objects in the Universe, with ages of 10 billion years or more. No one knows how they originate or where they come from or how they stay together.
Open clusters disperse rather quickly yet globulars seem to remain tight forever.
The Great Hercules Globular Cluster, which contains an estimated one million stars, is the most prominent globular in our northern skies. It is visible to the naked eye on clear, dark nights. In most cases, yes. Some pictures however are taken with filters to highlight otherwise difficult features to see such as nebulae, elemental gas clouds, interstellar dust, etc. These doctored pix are permeated with an obvious greenish or reddish glow.