Ok well this is a common question, and the answers are a little mixed. Some refer to indie as a particular style of music such as the indie-rock and indie-pop genres. Bands fitting into this genre include Coldplay, Radiohead, etc. But I’ll give you the definition that we use on this site which is music that is produced independently of the major record labels. Being independent generally means a lower budget so the bands tend to have a smaller fan base, and do a lot of the graft work themselves, however this does not generally result in lower quality music. Staying independent gives indie artists more freedom to make the music that they want to make, and their fan base while smaller, are often more dedicated as they see themselves as going against the mainstream, and they realise just how much their support means for the artists.
The trend for artists to stay independent seems to be growing. In the past, getting a record deal with a major label was the only real way to get your music heard. However the rise of the internet and social media gave artists the ability to promote their music on a tight budget and keep a much larger percentage of the resulting profits.