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Every ad platform — Google, Meta, TikTok, Reddit, Microsoft — wants to know two things: did anyone buy? and which ad caused it? They learn this from tiny scripts on your site that report back when something important happens. Tag Manager is the place you organise all those scripts.

A pixel is just a small bit of code (originally a 1×1 transparent image, hence the name). When it loads in someone's browser, the ad platform records a visit, a purchase, a sign-up — whatever you've told it to listen for.

• Meta Pixel tells Facebook/Instagram who's on your site.
• TikTok Pixel tells TikTok the same.
• Google Ads tag tells Google Ads when a conversion happens.
• GA4 tag sends behaviour into Google Analytics.

Instead of pasting each one directly into your site, you install GTM once and then add all the pixels from inside GTM. Much safer, much faster, no developer for every change.

GTM is a container that lives on your site. Inside it you add tags (the pixels) and decide when they fire (triggers). You preview your changes before going live, and every publish creates a version you can roll back.

  YOUR SITE  →  GTM container  →  GA4
                            →  Google Ads
                            →  Meta Pixel
                            →  TikTok Pixel
                            →  whatever you add next

• GA4 = the report card. Tells you what users did on your site (pages, products, purchases, retention).
• Google Ads / Meta / TikTok = the auction houses. Each needs its own conversion signal to learn which clicks turned into sales.
• GTM = the central control panel that feeds all of them from a single dataLayer.

• Tag = the script that does something (send a conversion).
• Trigger = the when (page load, button click, custom event).
• Variable = the data (order value, product ID, page URL).

A tag with no trigger does nothing. A trigger with no tag does nothing. Variables feed both.

The dataLayer is a list your site keeps so GTM knows what's happening. When someone adds a product to cart, your site pushes an entry into it. GTM is listening, sees the push, and fires whatever tags you've connected to it.

// Your site pushes this when someone adds to cart:
window.dataLayer.push({
  event: "add_to_cart",
  ecommerce: {
    currency: "GBP",
    value: 29.00,
    items: [{ item_id: "SKU_001", item_name: "Resistance Bands", price: 29, quantity: 1 }]
  }
});

That's it. You don't need to understand the JavaScript yet — just know that it's how your site tells GTM "something happened, here's the data".

1. Sign in to tagmanager.google.com and create a container.
2. Install the two snippets it gives you on your site (we keep them in Codes).
3. In GTM, click Tags → New → Google Tag and paste your GA4 Measurement ID (G-XXXXXXXXXX).
4. Set the trigger to All Pages.
5. Click Preview, browse your site, confirm the tag fires, then click Submit.

That's a working GA4 install. The rest of this hub is everything you'd add on top.

Tag

A script that does something — sends data to a platform.

Trigger

The condition that fires a tag (page view, click, custom event).

Variable

A piece of data used in a tag or trigger.

Container

Your GTM workspace. One per site, ID looks like GTM-XXXXXXX.

dataLayer

A JS array on your page that your site pushes events into.

Event

A named thing that happened (e.g. add_to_cart, purchase).

Pixel

Old word for a tracking script — Meta Pixel, TikTok Pixel etc.

Conversion

A valuable action you're paying ads to drive (purchase, lead).

GA4

Google Analytics 4 — Google's current analytics product.

Measurement ID

Your GA4 stream ID, looks like G-XXXXXXXXXX.

Enhanced Conversions

Hashed first-party data sent to Google Ads to improve match rate.

CAPI

Conversions API — server-to-server conversions (Meta, TikTok).

sGTM

Server-side GTM — a GTM container running on your own server.

Consent Mode v2

Google's framework for adjusting tags when users decline cookies.

Key event

GA4's term for a conversion event.