A lot actually. A private event is something which is, well, private to you and you would like to invite only a select group of people.
A public event is something which is attended by a lot of people. Saying this, there is nothing stopping you from making a private event - a public one and vice versa.

However, on ActiveCiti, the main differences between these two types of events are:

Private Events
Public Events
You can however, invite people in both the types of the events and in a public event -- the invitees will see more details than a non invitee who is viewing the page. The invitee will have access to the public forum and also the details of the organiser.

Good candidates for private events would be birthday / anniversary parties, movie outings, picnics, class reunions, etc.
Events such as rock shows, seminars, etc. will come under the public events category.

While creating your own event, ActiveCiti offers you the following features:

When you create an event, we allow you to upload two images to complement your event page.
One is the event banner (recommended dimension of 700px x 100px) and the other is the main image.
If the main image is greater than 300px x 300px, it is resized and the larger version is available on clicking the smaller thumbnail.
This feature makes it great for uploading posters and brochures which provide additional information about the event.

There are many websites like Google Maps, Wikimapia, etc. which allow you to mark areas or create maps providing you with a link which you can distribute to let others know of a particular place.
Thus, you can use these services, mark your event location and paste the respective link here.
Visitors coming to your event page will get a link which will open up the map to give them better directions to the location of the event.

E.g. Link to Taj Mahal, Agra
Using Google Maps is: http://maps.google.com/maps?f=q&hl=en&q=Taj+Mahal&ie=UTF8&ll=27.174765,78.04239&spn=0.00199,0.006781&t=h&z=18&om=1
Using Wikimapia is: http://www.wikimapia.org/#y=27174904&x=78042068&z=18&l=0&m=a&v=2

The private discussion forum is available on the create event page and as such, is available only to the event owner and the organisers.
This forum can be used to discuss event details with the other organisers, just leave messages or assign responsibilities ...

This will only happen when you are not the owner of the event.
Only an owner can add other organisers to the event. Organisers, cannot add other organisers.

Only invitees who are registered members of ActiveCiti, can be organisers to your event.
So, if there are people who you want to make organisers, but cannot as they are not members, ask them to sign up with the same email address you invited them.

Their new account will contain all the previous event invitations and you will be able to give them organiser privileges.

Yes. Though this is not enabled by default. You will have to set this option from the Guest List page.

The "Email Invitees" feature allows you to send emails to your invitees from within ActiveCiti itself.
You can also filter out the invitees you want to notify -- by their responses.

This is a good way to remind people who have not yet responded that the event starts in a couple of days and you are waiting for their response.

Event tags are available only for public events.
Event tags are basically keywords which describe the event and make the event more easily searchable when someone is trying to search for an event from the public events page.

When an event is created on ActiveCiti, the URL for that event is pretty cryptic and something like:
http://www.activeciti.com/public/display_event.aspx?id=B87CF8A6-3CA3-40c3-858E-01AE672DE0B8

A friendly URL generates a more easy to use URL which you can distribute verbally.
Hence, a friendly URL will be somewhat like: http://events.activeciti.com/johndoe_bday

The width of the banner image on the public display page cannot be more than 700px. Hence, when it is, we automatically resize it to maintain the page layout.
There is no restriction on the main image that you upload.
However, the space reserved to show the image on the page is 300px x 300px.
So, whenever the main image is greater than this size, a smaller (resized) version gets displayed on the main page instead and clicking on this image will popup the larger image.

This feature is excellent for uploading images of the event poster or the event brochure.

For best results, the banner image should not be greater than 700px x 100px.
There are no restrictions on the image size of the main image.
However, the images cannot be greater than 512 kilobytes in size.

When you create and event, you can go back to the event edit page. Information on who has responded what -- is displayed on the top right corner of the page under the Guest List.
The different icons symbolize the response of the invitee.

If you have enabled the option which allows guests to see who else is invited, this information is also available from the event display page.

An event shows up on the calendar when you create it or are invited to one.
Past events are also shown on the calendar. However, cancelled events are not shown at all.
If you had been invited to a particular event and then removed (or the event gets cancelled) so that you are no longer invited, the event will no longer show in the calendar.

When you cancel an event, you will no longer be able to access any of the event details and the event is deleted from the database.
Only an event owner may cancel an event (and not the organisers).
Once an event is cancelled, all the invitees get an email notifying them of the same.

When you add one of the invitees as an organiser, he / she gets access to the event edit page.
The organiser can do the following things:

The two things that an organiser cannot do are:

When you remove an invitee from your list, he / she gets an email informing them that the event has been cancelled (for them). The respective event is removed from their calendar and they can no longer access the event page (for a private event) or take part in the discussion.
If the said invitee was also an organiser, he / she is removed from that list too.
Also, the removed invitee will no longer get any event update emails.

When you remove an organiser, he / she still remains invited to the event, but will no longer be able to access the edit event page.
Hence, they lose all their organiser privileges.
However, as they are still invited, they can still access the event display page like all the other invitees.

The organiser can do the following things:

The two things that an organiser cannot do are:

You can remove the organiser in the following manner:

The removed organisers will get an email telling them that they are no longer organisers for the particular event and cannot access the edit event page any longer.

No.
His / Her status turns to a normal invitee and he / she can still continue to do the things an invitee can.

Private discussion forums are meant to discuss "insider", organiser specific topics which everyone does not need to know about.
Hence, this is available only on the edit event page and accessible only by organisers and the event owner.

The public discussion forum is open to all the invitees and can be used to discuss other general things which apply to everybody else.