Sierra Entertainment, formerly known as Sierra On-Line, was one of the major commercial producers of adventure games, the other one being LucasArts. The company is now defunct, its trademarks property of Vivendi Universal.
Sierra On-Line first started life as a side project of Ken and Roberta Williams, a couple who had fallen in love with the original Crowther and Wood's "Colossal Caves Adventure" (also commonly knows as "Adventure"). They had toyed with the idea of creating adventure games with graphics, and so they did - by today's standards the graphics were very poor, and to make allowances for the memory they used the parser was a far cry from the brilliance of Infocom games... but they still managed to come up with "Mystery House", "The Dark Crystal", and most notably "Time Zone", which has reputedly earned a place in the Smithsonian Institute as the "most complex adventure game ever created", a fame it lives up to.
However, it really started making its mark with "King's Quest", which had been comissioned by IBM, the new kid on the block. IBM wanted something to showcase its abilities - full-blown 16 colors, pseudo-3d environments (your character can even walk behind trees!), sound effects, and everything we all now know and love as Sierra's "AGI" period (AGI being the engine Sierra used for these games).
From that point onwards, Sierra started gaining more and more credibility and popularity, also making and producing non-adventure games. However, it's still hailed as one of the best (or at least, more prolific) adventure game company. Not surprising, since adventure games were its roots. Sierra also bought other smaller companies, notably Coktel Vision (of Gobliiins fame) and Dynamix (Willy Beamish and Betrayal at Krondor).
Nowadays, many Sierra games are frowned upon because they retain some aspects which were common at the time they were developed but are now (mostly due to LucasArts's policy of brilliantly working around them) considered poor design - walking deads and frequent, sometimes illogical deaths. Though there are many, many pros that outweigh these cons, these are considered essential flaws in gameplay nowadays, and newer players (who are also discouraged with the very old-school look and feel of these games) tend to gravitate instead to newer games like Broken Sword, or older and yet visually, audibly and gameplay-wise more appealing (Full Throttle and Day of the Tentacle being good examples of these latter - both Lucasarts titles, it should be noted).
Sierra's last games before being taken over by Vivendi are often hailed as "plain bad experiments into the new 3D fad". Gabriel Knight III, King's Quest VIII and Quest for Glory V are the examples most often cited, on the basis that GKIII's 3D was redundant and added nothing new to gameplay, KQ8 was a severe departure from the series' style so it could "cash in on the new fads", and QfGV was simply "bad". All these points are arguable, however, and these games still have defenders.
Famous series by Sierra include: