I'd like to create a content type "Enterprise" where user can define N services and, for every service, N tasks. I have already created the other fields for this content-type (name, description, ..., easy fields) but I don't know how to start with the services and tasks.
I'd like to be able to create services and tasks in the same form. What would be the best way to do it?
In case of use d6, you can use Flexifield, then after you install you should create a content type that should represent your subfields, and in your main content type you should create a new field, with Flexifield type, and then, define the it's a number of values of the field, and then, in the creation node form of your main content type, you`ll have a field (kind of group), that will have multiple subfields, and depending of the number of fields defined, you should have multiple "groups of subfields".
In drupal 6.x this used to be accomplished by using cck-3.x which had support for multi-group which is the behavior you're asking for.
In drupal 7.x this can be accomplish by using the field_collection module (http://drupal.org/project/field_collection) - THis module also requires the entity module.
Once you have that module set up the UI is a little different than jut adding a field and groups.
You're going to want to go to you "Enterprise" content type and add the new Field Collection field (right now the only display is hidden).
Once you added the field collection field :-). You will now need to go to field collection configuration screen over at admin/structure/field-collections. Here you control the fields within your collection.
So you should now see the new field_collection field you added, You will need to add the two new fields you want to that collection. In your case is the services and tasks fields.
Now you have a collection with two fields in it.
The weird thing is that in order to add collection right now you can't do it when creating the node. You will need to create the node and then you're provided with a "Add collections" link which allows you to add those "Services and Task".
Is a little wierd but it works. I'm sure in the future they're be more widgets available but for now thats whats available in drupal 7.
Hope this helps.