Following are the different types of forms used in Microsoft SharePoint’s workflow :

• Association and Initiation forms:
  Before each workflow begins, users are presented with association and initiation forms to complete. These forms can be used to ALLOW users to set parameters and other information for the workflow AHEAD of time. Initiation forms discuss how the workflow relates to a specific SharePoint item, WHEREAS association forms address how the workflow applies to a given list, library, or content CATEGORY.
  Administrators are presented with association forms when they initially select to associate a process with a specific list, document library, or content category. You can use association forms to allow an administrator to set workflow parameters, default values, and other information for items on the list, library, or content type with which the administrator is associating it.
  When users begin a workflow on a specific SharePoint item, they are presented with an initiation form. Initiation forms can be used to allow users to modify or append the association parameters provided by administrators, as well as to specify extra parameters or information about the workflow as it relates to a specific SharePoint item. Initiation forms aren't required for all workflows.
  The initiation form and the association form can be the same. Using the same form for each workflow form, for example, allows administrators to define some default values during workflow association, then let the user who begins the workflow instance on a specific item review and replaces those default options.
• Modification forms:
  You might wish to give your users the ability to CHANGE the workflow while it's running on an item. For example, you could wish to give a user the ability to delegate a task to someone else or even add a new activity to the workflow. Modifications are the options you give users to adjust the process while it's executing on an item. You can construct modification forms that allow users to provide modification parameters.
• Task forms:
  For each task in your process, you can provide custom forms. However, because tasks are SharePoint items with a content type assigned to them, the task type's custom forms are determined by the content type. All SharePoint task types are allocated content types by default. If a task type does not have a content type assigned to it, the task type is given the Task base content type. The WorkflowTask foundation content type must be used for all task-type content types. A content type's custom new, edit, and display forms can be specified. When displaying a workflow task, Microsoft SharePoint Foundation uses any custom forms you've created for the relevant task content type.