Maybe this will help...
Blu-Ray discs use a slightly different format [e.g. UDF 2.5] so Windows needs to be able to read that format -- that's only an issue in XP, where Aiseesoft Blu-Ray apps will add a hacked Toshiba driver. You do need a Blu-Ray drive to read actual BD discs, though Blu-Ray on DVD [which is spec] will work with regular DVD drives as long as Windows reads the Blu-Ray UDF format.
Retail Blu-Ray discs have loads of DRM, with menus etc. programmed using Java -- to play it completely, i.e. all features, you'll have to buy a player [unless one came with your Blu-Ray drive] because the developers that make these players have to pay licensing fees. If or as possible try before you buy because performance & features vary, sometimes quite a lot. Many will not play Blu-Ray in folders on your hard drive.
Many claim that a fair number of existing media players will play Blu-Ray, but playback is actually more at a beta level -- expect glitches from time to time. And as above don't expect them to fully play anything but the movie. Also, some retail Blu-Ray discs use VC1 video rather than the more common AVC/H.264 -- not every media player will handle that. And finally do not expect something like VLC to do anything about a Blu-Ray disc's DRM.
Aiseesoft's Blu-Ray Player basically works like their Blu-Ray copy &/or ripping apps to by-pass a retail disc's DRM. Like the free players others have mentioned, the Aiseesoft Blu-Ray Player [& similar players from other companies] does not play all features. For Blu-Ray discs it's IMHO heads & shoulders above something like VLC, but compared to other commercial [i.e. not free] players it comes down to price & how much you want to play a disc's features beyond the movie itself. Today obviously cost is not a problem.
When it comes to the actual quality of playback, AFAIK the only free players with much of anything in the way of enhancements are MPC-HC [ http://www.videohelp.com/tools/Media-Player-Classic-Home-Cinema ] & MPC-BE [ http://www.videohelp.com/tools/MPC-BE ] -- otherwise if that's what you're after you'll have to get something like PowerDVD, & unless you spot it on sale, the versions that'll handle Blu-Ray can be a bit expensive. If you want or need those enhancements, which may help a lot on some systems, I *think* the most common route is to copy the video from a Blu-Ray disc unaltered into a MKV container file, then use one of those 2 players, but it's not something I do or have tried so research 1st at sites like videohelp.com . [If it's legal where you live, & if you're going to copy a Blu-Ray disc you own to your HDD to bypass DRM, writing the copy as a MKV file simply saves some hassles.]