Trello – What’s the difference between subscribing to a Board vs subscribing to a card

As far as I know it's the same thing except:

• You will automatically subscribe to new cards added to the board.
• You won't see the eye symbol on every card if you're subscribed to the board.

From this card on the trello development board:

Board and List Subscribe

People are looking for a way to get notified of all actions on a given card, list, or board, without being assigned to it. Subscribing will do that.

You can also subscribe to a list from the list menu (upper right button). You'll get notified of all actions that happens to cards in that list. The list will show a subscribe icon, but cards in that list won't show the icon unless you've subscribed to them.

You can subscribe to a board from the board menu. It will add a subscribe icon to the board title. Like lists, it's not going to add an icon to every single card on the board.

Permissions: You won't be able to subscribe to something unless it's on a board you can see. If you switch a board from public to private, people subscribed to it won't get notifications and it won't show up in your subscriptions list.

And here's from the trello blog introducing this feature:

Subscribing to a card lets you get notifications as if you were added to it. When we launched it back in May, people loved it. The biggest feedback we received was that people wanted the ability to subscribe to whole boards and individual lists. We admit we were a bit hesitant — that’s a lot of notifications, at least for us. But there are some cases where it makes sense. Sit back and imagine…

Let’s say you are on a Trello board shared by two other roommates. It’s filled with household tasks, upcoming bills and rent, grocery needs, etc. There’s not much activity so you want to know about everything that’s going on. Subscribing to every card would be a… chore, so you subscribe to the whole board. But you find that you don’t really need to know that Steve finished washing the dishes. You really only need to know about what to do next, so you just subscribe to the “New” list. Next thing you know, you’re out grocery shopping and hey look, you get a push notification on your iPhone or Android device. ”Get butter” was added to the list. You pick up a dozen boxes and return home where everyone thanks you profusely.

Anyway, you get the idea. (Also you might want to cut back on the butter…)

We’re excited to announce the launch of board and list subscribe. We can’t guarantee that it will make you the hero of your household, but we hope you find it useful. How does it work? To subscribe to a board, just click the “Options” button, then select “Subscribe”. To subscribe to a list, click the menu button in the list header, and select “Subscribe”. Now you can sit back and see changes without having to click through to every board.