The term "mixtape" itself is somewhat of an enigma. Basically it means, "a tape of mixes" but in many cases, it also means "a tape of remixes". Any compilation of songs that is not an album could theoretically be called a "mixtape".
A "Mixtape" is usually an album mainly made up of Remixes, B-Sides, unreleased, and exclusive tracks. They are mainly made by DJs, and are released to promote the DJ, and all the other artists that are featured on the Mixtape. These mixtapes normally use music sampled from other artists' songs, which are not normally cleared for use, but are overlooked by most music authorities, including those who's music is sampled. Many mixtapes contain mostly Explicit content, and very rarely do people compile mixtapes of Edited material.
For concert bands, mixtapes are normally either sold at concerts, at the artists'/DJ's websites, or at underground music stores, normally priced at only a fraction of the cost of a normal CD, typically ranging from $4.00 to $10.00 apiece. While other, more rare or specially made Mixtapes can range from the price of a normal CD, to even more. Mixtapes are also typically online, sometimes for free download, while their physical packaging normally consists on only the CD, a Slim-Jewel case, and front artwork. Although, some more special Mixtapes, come in standard Jewel cases, front and back artwork, a booklet, and maybe other special items, like a poster or a bonus DVD, although this is rare.