There are as many types of medical school curriculums as there are medical schools, but there are a few basic categories:
Block Scheduling: Yes, this means that one class is covered at a time, but there is more than one type of block scheduling. Some are "system-based" curriculums where you take classes that are divided up by organ system. For example, at a systems-based school, your one class in September may cover circulation/blood, the brain, or the reproductive system. That class would include all of the anatomy, cellular physiology, pharmacology, etc that you need to understand the organ system.
Other schools still use block scheduling, but divide by type of material rather than organ system. At these schools, you would have separate classes exclusively devoted to things like anatomy, histology, pharmacology, etc. During a pharmacology class, for example, you would cover the pharmacology of ALL organ systems.
Whether these block courses are integrated really depends on your definition of integrated. For example, the first type is integrated with respect to organ system - and this is what most people mean when they talk about an integrated curriculum.
Non-Block Scheduling: Some people refer to this as the traditional curriculum. These schools teach multiple classes simultaneously - students may be taking gross anatomy, biochemistry, and histology all at the same time. However, this can be deceptive because some block courses can cover the material of multiple classes as well but collapse it into one class (for example, at my school one of the block courses covers gross anatomy, embryology, and imaging at the same time).
PBL: Problem-Based Learning is one type of small group learning system that some schools use. During PBL sessions, students are usually presented with some case that they then discuss/diagnose/analyze. The idea behind PBL is that small groups of students have the chance to directly apply what they are learning. PBL is usually used in addition to lecture.
Non-PBL Small Groups: Many schools use small groups for things other than problem-based learning. One major example is physical exam class where students are divided into small groups to practice. Schools may also use small groups as journal clubs where students discuss papers.
Clinical courses: These courses are usually not counted as part of the general "block" or "traditional" curriculum. Some schools provide lectures on topics in medicine such as public health, sensitive patient issues (abuse, drug use, etc), and some don't. These courses may include a clinic component where MS1 and MS2 students go and work in a hospital or clinic a few times each week or month. This is usually what schools are talking about if they mention early clinical exposure.
In general, schools mix and match from the components I have listed above. Some may even use a "non-systems" approach for MS1 year and a "systems" approach for MS2 year. Each school does things its own way, but hopefully this helped!
