My definitions for Casual, Semi-casual, and hardcore raiding would be as follows:
Group makeup: Fixed, generally same people, but at least same class distribution every time, focusing on optimal setups.
Goal: Progression. Standing may or may not be a part of it. Guilds who start raiding late into an expansion are hardly going to have any shot at standing, but doesn't mean they don't want to progress.
Raidmember responsibilities: Thorough class research to optimize DPS, such as casting order, rotations, talent specs, gear, mastery, glyphs, enchantments, etc etc. Other raidmembers will be doing this and will expect you to do the same. Have appropriate mods installed. Also, thorough research of the encounter, watching videos, reading walkthroughs, writeups, tips, etc so that even if it's your first time, you understand the theory of what you need to be doing, even if you need practice.
What to expect on the raid: For new raidforces - grind, grind, grind, wipe, wipe, wipe. The goal is to learn and beat each encounter as soon as possible. So if that means spending 3-4 hours constantly wiping as you learn a new encounter, then that's what you're going to do. For raidforces that have been around awhile - kill, move, kill, move, kill, move. The raid has done these encounters 50 times already, so the goal is to kill as much as possible, so be prepared, try to keep up, and for godsakes no stealth AFK
Group makeup: 60-80% fixed. Generally these have a core of consistent raiders, with the rest being filled out by a group of rotating people who just can't attend every raid session. Groups are built to be as optimal as possible, but obviously don't sweat having one or two redundant classes here and there.
Goal: Gearing up and progression. Gearing up this force takes longer, both because of the larger raid pool, but also with some of the raidforce having odd attendance, it may take awhile to get them geared, depending on how frequently they can attend. Progression is the ultimate goal, but considering the harder encounters require better gear, gearing necessarily comes first. While progression, as a goal, is similar to that of a hardcore guild/group, the reasons are not. To my mind, for hardcore, progression is the means and the end, but for semi-casual, progression is the means to see something new. "We've killed mob A 15 times now, it would be nice to work on and kill mob B for a change"
Raidmember responsibilities: You don't need to be optimized gear and skillwise to within an inch of your life as with a hardcore group, but you should at least know your class fairly well as to how you should generally spec and have some enchantments/glypsh, etc. The core members will generally be optimized closer to the hardcore framework, mainly because since they attend every raid they will be more of that mindset. Even so, for the most part (I like to think), they are more willing to work with the less frequent attendees with guidance, instruction, and accepting of sub-optimal gear. Should try to understand the boss encounters beforehand, but instructions can and usually are given everytime, as new people showup, or to refresh the minds of people who've been away.
What to expect on the raid: Mostly rehashing content that's been killed already, generally to gear people. New encounters will be worked on, but it's usually not the main focus. The time spent working on new encounters is generally dictated by the raidforce. If the raidforce can kill the old stuff quickly enough, will work on the new. Fair amount of time may be spent on instruction, either on specific encounters, or general raiding.
Group makeup: Only requirements are the obvious ones: sufficient tanks, heals, etc. Again, may build the most optimal raidforce with whats given, but it's possible nobody in the raid knows what that is.
Goal: Gear, chat, have a beer, not necessarily in that order. People in this will have the most varied reasons for doing it. Some will actually want a shot at some gear, others just want to chill or chat with guildies.
Raidmember responsibilities: Listen to your raidleader. While this even is probably more social event than raid, it still IS a raid, so try to listen to directions at least a little bit, hehe.
What to expect on the raid: Nothing. It is a casual raid, so don't go in expecting to go full bore working on defeating some encounter. You're only going to get frustrated and angry and may say things you probably shouldn't. Take a deep breath, put on some Spyro Gyra, and accept any gear that you get as mana from heaven
As a caveat, I will say the the goal for all these types is to have fun, but broadly speaking, the definition of fun varies from type to type. Just make sure your personal goal and style matches that of the group you're joining. Oh yeah, and have fun, whatever that is