There are more threads on this than hydrogen atoms in the Milky Way, but here's as comprehensive and unbiased an answer as I can give you.
DO is a degree for osteopathic medicine and is functionally equivalent in terms of scope of practice and licensing to an MD degree. DO schools are generally newer than MD schools and are less likely to be affiliated with large academic medical centers. This means less access to research and connections in more research heavy-fields, as well as home departments for these fields. DOs currently have their own residency match (AOA match) that is representative of all specialties, but by the time you are matching, DO and MD residencies will be merged - the effects of this merger have been speculated about on plenty of other threads, so I will not go into that here. DOs can match into MD (ACGME) residencies if they take Step 1 in addition to the COMLEX, but they will still be at a disadvantaged compared to MDs with similar profiles.
DOs can theoretically match into any specialty that MDs can, but in practice they are more limited due to anti-DO bias that exists at many academic medical centers, especially within certain specialties. One of the reasons this bias exists is because clinical rotations are not standardized in quality across DO schools, so program directors may be hesitant to take students from a school that may not be a known quality. However, many community and non-top tier academic programs are still readily accessible to DOs in most specialties.
DOs have an added curricular component called OMM, a practice with questionable scientific evidence, though it is not something that you devote the majority of your learning time to. As some might put it, it's just another "tool in the box".
More DOs go into primary care than MDs. According to wikipedia, about 2/3s of all DO graduates go into a primary care field while 1/3 of all MDs do. This gap is probably even bigger because MDs are more likely than DOs to match to academic medical centers and thus have more opportunities to specialize, likely cutting the number of MDs that go into true primary care by 33-50%.
The tradeoff is that DO schools have generally less competitive MCAT/GPA/research profiles than their MD counterparts and have a policy called grade replacement where they only count the more recent of two grades for the same class, whereas MD admissions count both equally. This allows people with a less than stellar academic record to have a chance at redemption. Additionally, the median DO school MCAT is around a 27 while the median MD MCAT is around a 31, so there's a bit more leeway there too. However, this gap has been closing in recent years.
In terms of matching difficulty for DOs, here is generally what I have observed (people more familiar with this process, please correct me):
Easy: pediatrics, internal medicine, pathology, family medicine, PM&R, psychiatry
Medium: OBGYN, emergency medicine, anesthesiology, neurology
Hard: general surgery, radiology
Extremely hard (do not at all count on matching into these as a DO): neurosurgery, plastic surgery, orthopedic surgery, ENT, ophthalmology, dermatology, radiation oncology, urology
That's about it I think. People more familiar with DO schools, please correct any mistakes!