Trip & Conditions Reports
Conditions reports tend to be brief and just focus on things like snow, water levels, obstacles, mosquitos, and things of consequence. Because they're easier to write, they tend to be up to date and are useful when you have an idea of where you want to go but want to doublecheck conditions (or possibly postpone a trip!).
More traditional trip reports are written in more of a narrative journal form and will focus on more static aspects of a trip like routes, as well as the subjective experience of them. They're better for trying to see if you'll be interested in an area or to check if the route is within your skill and comfort levels.
Conditions Reports
The lack of archival ease of use is balanced out by the lack of friction so there tends to be a lot of briefer but up to date posts at Sierra Nevada Trip Reports & Current Conditions. Trip reports will have some beta or route descriptions, but are focused mostly on conditions.
If you are doing something very popular (or something that shares a pass or two), there are dedicated PCT, JMT & the JMT NoBo Info Hub, Rae Lakes loop, SEKI High Sierra Trail, Whitney, Tahoe Rim Trail etc Facebook groups that should have very good information on those specific areas. Many often have yearly groups as well, which are probably more informative and less navel-gazey nostalgia. There's a few general Sierra Nevada groups that will have trip reports, but there tends to be a lot of noise to signal. Eastern Sierra Hiking & Backpacking is one of the more informative ones.
There also subreddits for the PCT (very active & has weekly conditions threads), JMT (fairly quiet), SEKI (fairly active), Yosemite (fairly touristy), Eastern Sierra (pretty much dead).
Backcountry rangers file reports of trails they've been on and also get beta from hikers that have reported back after a trip. Calling a wilderness center and getting beta can be very useful. Yosemite updates their Wilderness Conditions page with information gathered from backcountry rangers on a somewhat regular basis. SEKI has a similar Trail Conditions page.
Usefulness can vary greatly but it's heavily used and searching for a trail or peak on All Trails can be a good way to see what people have encountered recently.
Postholer google map overlay lets you pull in a ton of info on PCT segments.
Classic Trip Reports
High Sierra Topix is a classic message board with a lot of information buried in old threads. If you want a little bit more info than what Secor has for a cross country pass or are interested in a basin this is a good place to start. There's yearly conditions threads, though the community is more focused on traditional narrative trip reports & armchair backpacking.
Another good source for XC trip reporting is climber.org. The interface is a bit dated, and there's often more technical XC beta included in trip reports than backpackers will find useful, but a lot of interesting terrain is on the way to a peak somewhere and there tends to be detailed descriptions backed by photos.
Summitpost is also geared more towards climbers & mountaineers vs backpackers and hikers, but as with climber.org there's overlap and looking up a peak can be a great starting point in planning.
The Sierra Club SPS Trip Reports have a lot of peak based entries.
Bob Burd's personal site is very peak focused, but included detailed information on approaches that you could extrapolate from.