TSR (short for Tactical Studies Rules) was formed by E. Gary Gygax and others to publish the rule set for Dungeons and Dragons. After several missteps, the company became debt laden and Gygax lost control.
However the new management failed to reverse the situation and the company's debt continued to increase. When combined with other problems such as poor sales in new lines and lax stock control the company came to the brink of insolvency.
TSR also published a number of rules sets for other periods including the American west, WWII, and medieval (Chainmail).
After initial success faded the company would often turn to legal defenses of what it now regarded as its intellectual property. In addition to this there were several legal cases brought regarding who had invented what within the company and the division of royalties. These actions reached their nadir when the company threatened to sue individuals supplying game material on internet sites. The company was widely perceived to be attacking its own customers.
TSR -- acronym for 'Terminate and Stay Resident'. TSR programs ran on MS-DOS systems, and were loaded either during the boot-cycle of operating system (being explicitly loaded in either the AUTOEXEC.BAT or CONFIG.SYS scripts) or later, at the user's request (for example, Borland's Sidekick). Due to the delay that it normally took to load the program, it was deemed to be more efficient to leave the program in memory than to reload it again each time it was needed.
Most TSRs are now incorporated into the operating system, and reside in memory, but not as independent programs.