The SharePoint Activity Feed rendering is controlled by templates. These templates are used within the ActivityTemplate class http://msdn.microsoft.com/en-us/library/microsoft.office.server.activityfeed.activitytemplate.aspx which is responsible for controlling the rendering.

The OOB templates are held within a resources file located:

C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\Resources\osrvcore.resx

This contains the following templates:

Keyname: “ActivityFeed_ProfilePropertyChange_SV_Template”
Template: {Publisher} updated profile.<br/>{Name}: {Value}

Keyname: “ActivityFeed_ProfilePropertyChange_MV_Template”
Template: {Publisher} updated profile.<br/>{List}

Keyname: “ActivityFeed_Birthday_Reminder_SV_Template”
Template: {Publisher} is celebrating a birthday on {Name}.

Keyname: “ActivityFeed_Birthday_Today_SV_Template”
Template: {Publisher} is celebrating a birthday today.<br/>Wish {Publisher} a happy birthday!

Keyname: “ActivityFeed_WorkplaceAnniversary_Reminder_SV_Template”
Template: {Publisher} is celebrating a {Value} year workplace anniversary on {Name}.

Keyname: “ActivityFeed_WorkplaceAnniversary_Today_SV_Template”
Template: {Publisher} is celebrating a {Value} year workplace anniversary today.<br/>Wish {Publisher} a happy anniversary!

Keyname: “ActivityFeed_ColleagueAddition_SV_Template”
Template: {Publisher} added a new colleague.<br/>{Link}

Keyname: “ActivityFeed_ColleagueAddition_MV_Template”
Template: {Publisher} added {Size} new colleagues.<br/>{List}

Keyname: “ActivityFeed_TitleChange_SV_Template”
Template: {Publisher} has a new job title.<br/>{Value}

Keyname: “ActivityFeed_ManagerChange_SV_Template”
Template: {Publisher} has a new manager. <br/> {Link}

Keyname: “ActivityFeed_BlogUpdate_SV_Template”
Template: {Publisher} published a new blog post.<br/>{Link}

Keyname: “ActivityFeed_DLMembershipChange_SV_Template”
Template: {Publisher} has a new membership. <br/> {Link}

Keyname: “ActivityFeed_DLMembershipChange_MV_Template”
Template: {Publisher} has {Size} new memberships. <br/> {List}

Keyname: “ActivityFeed_SocialTaggingByColleague_SV_Template”
Template: {Publisher} tagged {Link} with {Link2}.

Keyname: “ActivityFeed_SocialTaggingByColleague_MV_Template”
Template: {Publisher} tagged {Link}.<br/>{List}

Keyname: “ActivityFeed_NoteboardPosts_SV_Template”
Template: {Publisher} posted a note on {Link}.<br/>{Value}

Keyname: “ActivityFeed_SocialTaggingByAnyone_SV_Template”
Template: {Publisher} tagged {Link} with your interest.<br/>{Link2}

Keyname: “ActivityFeed_SocialRatings_SV_Template”
Template: {Publisher} rated {Link} as {Value} of {Name}.

Keyname: “ActivityFeed_SharingInterest_SV_Template”
Template: {Publisher} shares an interest with you. <br/> {Value}

Keyname: “ActivityFeed_SharingInterest_MV_Template”
Template: {Publisher} shares {Size} interests with you. <br/> {List}

This contains the following template names:

Keyname: “ActivityFeed_ChangeMarker_SV_Template”
Template: Previous Gatherer Run

Keyname: “ActivityFeed_Status_Message_SV_Template”
Template: {Publisher} says “{Value}”.

Keyname: “ActivityFeed_ProfilePropertyChange_Type_Display”
Template: Profile update

Keyname: “ActivityFeed_Birthday_Reminder_Type_Display”
Template: Upcoming birthday

Keyname: “ActivityFeed_Birthday_Today_Type_Display”
Template: Birthday

Keyname: “ActivityFeed_WorkplaceAnniversary_Reminder_Type_Display”
Template: Upcoming workplace anniversary

Keyname: “ActivityFeed_WorkplaceAnniversary_Today_Type_Display”
Template: Workplace anniversary

Keyname: “ActivityFeed_ColleagueAddition_Type_Display”
Template: New colleague

Keyname: “ActivityFeed_TitleChange_Type_Display”
Template: Job title change

Keyname: “ActivityFeed_ManagerChange_Type_Display”
Template: Manager change

Keyname: “ActivityFeed_BlogUpdate_Type_Display”
Template: New blog post

Keyname: “ActivityFeed_DLMembershipChange_Type_Display”
Template: New membership

Keyname: “ActivityFeed_SocialTaggingByColleague_Type_Display”
Template: Tagging by my colleague

Keyname: “ActivityFeed_NoteboardPosts_Type_Display”
Template: Note Board post

Keyname: “ActivityFeed_SocialTaggingByAnyone_Type_Display”
Template: Tagging with my interests

Keyname: “ActivityFeed_SharingInterest_Type_Display”
Template: Sharing Interests

Keyname: “ActivityFeed_ChangeMarker_Type_Display”
Template: Gatherer Change Marker

Keyname: “ActivityFeed_SocialRatings_Type_Display”
Template: Rating

Keyname: “ActivityFeed_Status_Message_Type_Display”
Template: Status Message

The curly brackets are then replaced by the UI web part rendering code.

Hopefully this sheds a little bit more light on the internals of the activity feed internals.