I'm going to assume here that you've read The Lean Startup by Eric Reis and are looking for what to measure to prove you're moving in the right direction.
As Daniel mentions in his answer one of the key messages from the book is to avoid Vanity Metrics, that is to say metrics which look good but don't actually measure the success of your product.
For the sake of this answer let's say you have a website selling a piece of software which users pay monthly for. For me the obvious vanity metrics you should avoid at all costs are:
- Number of visits to the website
- Number of inquiries about the software
I'd also suggest that number of downloads and even number users who sign up for a month is fairly poor as what you want are engaged and active users.
The Goal is worth a read. There's a similar theme in there about the main character measuring the machine efficiencies rather than money made by the company. The book is a little out of date but the message is still powerful, this is a question about scope.
The short version, what you measure depends on what your goals are!
For example, let's say we want to add a Free Month feature to our website which lets users download the software and use it for free without handing over money. We want to measure whether this improves the uptake of our software.
In my opinion measuring whether the number of downloads increases is a fairly poor metric. Of course you're going to get more people downloading it, it's free! What you really want is more paying users.
A better metric would be how many paying users you have at any one time. If that number goes down after your free trial offer (and there are no other impacts) then the free trail clearly isn't improving sales and should be removed (ok, really what you should do in this scenario is find out WHY people didn't buy but stick with the example).
What you are trying to do is find a metric which represents happy, contented, and sticky customers.
Your metrics come down to ultimately what you're trying to achieve but I suggest you stay practical and think big to include the scope. Think about what is the goal of your product from a business point of view.
- Number of user accounts is poor
- Number of Active user accounts is better
- Number of Paying user accounts is best
As ultimately your goal is (most likely) to convert free users into profit.