In a comment to yesterday's post I was asked "What's the deal with academic conferences? Do you have to be within the correct academic circles to attend or is there some degree of public allowance?" I started to type up a reply, and then realized I had enough to say on the topic to warrant making a separate post on the topic.
Most academic conferences are open to anyone willing to pay the admission fee, which is often higher for people who aren't a member of the sponsoring organization. However, it is often worth purchasing a membership to the organization if you wish to attend their conference, as the amount you save on registration fees is larger than you'd spend on the membership, particularly if you are a student, since student memberships are usually quite affordable compared to the "professional" membership rate). If all you want to do is attend as a audience member, hear the talks, have coffee with the folk there, check out the info booths and see what is being sold in the booths offering stuff for sale all you need to is pay the registration fee and you are good to go.
However, If you want to share your research with others you need to look over the various sessions being offered, find one wherein your research fits the theme, prepare an abstract describing your research, submit it (and the abstract submission fee) before the deadline *and* register to attend the conference (and pay the registration fees, too). Often when submitting an abstract they ask if you prefer to do your presentation as a poster or a talk. However, even if you choose talk you might wind up doing a poster anyway; if they have too many talks offered for a session they will choose some of them to be posters instead. It is also possible that a session won't accept someone's abstract at all, but I have yet to see an example of this.
Another reason to attend conferences (at least in Geology, but possibly in other sciences, too) are the pre-, post- or mid- conference field trips. Most conferences offer 1 to 5 day trips to look at the highlights of local geology, with the trip led by people who have long worked in the area. These are excellent opportunities for networking, and, more importantly, they tend to showcase some of the best outcrops available in a region, so there are many opportunities to increase your photo collection of interesting geologic features, or to add to your sample collection (note: not all areas permit sampling—please listen to the rules at the start of the trip and respect them).
Short courses are another highlight of many conferences. There are often courses on speciality topics that are held at the conference because it saves the students the travel costs—much cheaper to attend both on one trip than to do to separate trips. Note that field trips and short courses are usually priced separately from the conference registration. Luckily, students who enjoy funding that includes conference attendance can usually attend courses and/or field trips without having to pay for them out of pocket (or, at least get reimbursed afterwards).
The final reason to attend conferences is the social aspect. This varies from one to the next, but I have seen offerings ranging from Ice Breaker Cocktail nights, to Conference Dinners, to Ceilidh Dances, to Choir rehearsals/performances to sporting events.