Battlestar Galactica is best watched in a definite progression so you understand the long story arcs.
1) Miniseries (2003) - Originally aired in two 2-hour parts (1 hr 30 min without commercials), presented on DVD releases as one 3-hour movie. Basically a 3-hour pilot. Region 1 DVD of Season 1 includes this on Disc 1; in other countries, it is available separately. While a recap is provided in the first regular-series episode, like the pilot of Twin Peaks, it is very necessary to watch the miniseries beforehand. Will be included in the "Complete Series" DVD/Blu-Ray set to be released on July 28, 2009.
2) Season 1 (2004/2005) - Contains 13 episodes, "33" through "Kobol's Last Gleaming, Part 2." Will be included in the "Complete Series" DVD/Blu-Ray set.
3) Season 2 (2005-2006) - Contains 20 episodes. Released in US as "Season 2.0" (10 episodes) and "Season 2.5" (10 episodes); elsewhere as "Season Two" (all 20). The US "Season 2.0" contains the aired (44 minutes) version of "Pegasus," while Season 2.5 begins with the extended version (58 minutes; highly recommended over the aired version) - releases in other countries only contain the extended version. Will be included in the "Complete Series" DVD/Blu-Ray set (including the extended "Pegasus").
4) Battlestar Galactica: The Resistance webisode series (2006) - Consists of 10 "webisodes" that bridge the gap between Seasons 2 and 3. Not 100% necessary, but certain aspects of Season 3 pay off much better if you've seen the webisodes. Not available (legally) outside of North America - available online or on Region 1 Season 3 DVD set (inexplicably among the special features on Disc 2, though the webisodes take place before any of the episodes in the season). Will be included in the "Complete Series" DVD/Blu-Ray set, though likely not in high definition despite being on Blu-Ray.
5) Season 3 (2006-2007) - Contains 20 episodes. Region 1 DVD contains both aired (43 mins) and extended (70 mins) versions of "Unfinished Business" (extended version recommended over aired version) - other releases do not contain extended version. Will be included in the "Complete Series" DVD/Blu-Ray set (including the extended "Unfinished Business").
6) Battlestar Galactica: Razor (2007) - 2-hour TV movie (officially the first 2 episodes of Season 4). Takes place during Season 2 with flashbacks to the miniseries and earlier, but should be viewed here, not during Season 2. Not 100% necessary, but it adds detail to certain aspects of the series. DVD contains aired (90 mins) and extended (103 mins) versions (extended version recommended over aired version). Another webisode series, Battlestar Galactica: Razor Flashbacks, was produced and released around the time of this movie, and all episodes are included on the DVD release, though many of them have been re-edited back into the extended cut of Razor. The remaining webisodes that aren't included in the extended cut are mostly superfluous and unnecessary (the first two webisodes, which aren't included in the extended cut, were directed by Wayne Rose instead of Felix Alcala, who directed the film and remaining webisodes), doubly so when one considers that Razor itself isn't entirely necessary, either. Will be included in the "Complete Series" DVD/Blu-Ray set (including the extended version).
7) Season 4, Part 1 (2008) - Contains 10 episodes. Also known as "Season 4.0." US DVD release contains Razor DVD as disc 1. Will be included in the "Complete Series" DVD/Blu-Ray set.
8) Battlestar Galactica: The Face of the Enemy webisode series (2008-2009) - Consists of 10 "webisodes" that take place between Season 4, Part 1 and Season 4, Part 2. Currently available to US residents on SciFi.Com (probably available illegally elsewhere to non-US residents). Function is similar to The Resistance. The events of this actually take place after the first episode of Season, Part 2 (or 4.5), though they were released before that episode. It doesn't really matter much whether you watch it before that episode or after (though I would recommend watching it after). Will not be included with the Region 1 Season 4.5 DVD/Blu-Ray, the "Complete Series" DVD/Blu-Ray set, or the Battlestar Galactica: The Plan DVD/Blu-Ray. Universal has publicly stated that they are aware of fan demand for a DVD release of this webisode series, and that they are "looking into" a future DVD release.
9) Season 4, Part 2 (2009) - The final 10 episodes. DVD and Blu-Ray to be released on July 28, 2009 in the USA and Canada (the region 1 DVD will be "Season 4.5," while regions will be titled "The Final Season" since "Season 4.0" was released there as "Season Four"). 3 episodes from this "season" will be extended on the Region 1 DVD (the originally aired versions will also be included): "A Disquiet Follows My Soul" (aired version 43 mins, extended version 53 mins), "Islanded in a Stream of Stars" (aired version 43 mins, extended version 63 mins), and "Daybreak," (aired version, 140 mins - including Part 1's closing and Part 2's opening credits; extended version, 152 mins). The extended cut of "Daybreak" will be presented as one triple-length episode on the DVD, as that's how the finale was initially structured, while the aired version will be split into two parts, as originally aired. Will be included as part of the "Complete Series" DVD/Blu-Ray set.
10) Battlestar Galactica: The Plan (2009) - Post-series prequel TV movie to be aired on SyFy in November. The movie will deal with the events of the miniseries and immediately thereafter from the Cylon point of view. It will be released on DVD and Blu-Ray on October 27, 2009, several weeks prior to its (as yet unanounced) SyFy airdate. The DVD and Blu-Ray will include the extended director's cut (112 mins, which includes graphic violence, sexual content and nudity), but will not include the TV cut (presumably 90 mins) to be aired in 2010. Is not included in the "Complete Series" DVD/Blu-Ray set. Despite rumors to the contrary, the DVD/Blu-Ray will not include The Face of the Enemy webisodes.
11) Caprica (2009/2010 - ?) - Prequel TV series set 58 years before the miniseries of Battlestar. An extended cut (92 mins, which includes nudity) of the pilot episode was released on DVD on April 21, 2009, but the TV cut (presumably 90 mins) will not be seen until the series' premiere on SyFy in January 22, 2010. The series deals with the creation of the Twelve Colonies' Cylons, and stars Eric Stoltz as Daniel Greystone, creator of the Cylons, and Esai Morales as Joseph Adama, father of Battlestar's William Adama, who is portrayed in Caprica by child actor Sina Najafi. As this is not technically part of Battlestar Galactica, it will not be included in the "Complete Series" DVD/Blu-Ray set.