Think of it as the same as a regular ER, except it's psych. You will triage, assess, stabilize, administer medications, and either admit them to your inpatient/medical facility, or send them on their happy way. Depending on what facility you're at and/or where it's located, you may also be a receiving unit for those under involuntary holds, so you may also get a chance to meet a wide variety of local law enforcement.
Like regular ERs, the acuity can vary: you have those who walk in because they to have their meds adjusted and want a new script....and then you have those who are having the psych equivalent of a MI. And then there's a whole range in-between.
The nice thing about working in the psych ER is that patients usually stay for 24 hours or less before they're either admitted, transferred or discharged. Occasionally you get a few patients who stay longer for whatever reason, but even those usually don't say more than a few days. Downside? Since the patients turn over so frequently you don't have days to try to build a therapeutic relationship with them: you need to be good at starting the groundwork for it ASAP. You also have to get used to dealing with a whole new set of faces--and lots of them--every day.
Personally, I love it. Some days are mellow, and some days it's like they opened the gates of Hell. Most days are in the middle...the pace is definitely faster than on most psych units, even ICUs.