Yes the word is effective. To create an effective app you should first decide who will be your primarily targeted users. What will be their age group. What the possible percentages will be shared between male and female. You also needs to have an idea what may be the income range of your users, the range may be wide or a narrow one. However an idea about it may be helpful.
Depending on the results above you will need to decide whether you need an Android App or iOS App or any other app. You may also choose frameworks like phonegap to develop the app. Using frameworks like phonegap, mobile apps build for one OS can be easily converted for another OS. In case of confusion you may ask the experts. One of the important thing is you need to decide an approximate budget for the app. Depending on the budget you may decide upto what extent the app will extend. You may discuss your idea of the app along with you budget with experts to plan the designs and functionalities of the app.
If you already have an website and you want to build an app for more user engagement then you already have an userbase(a ready set of users), you will need to promote the app among you users through your website and other mediums. The thing needs to be kept in mind that one should not blindly convert every thing that is displayed on a website into a mobile app. It needs to be understood that the screen size of a mobile / tablet is much different than that of a desktop or laptop. Thus the elements of an app needs to be displayed differently in an mobile application. Along with that a mobile device has some capabilities over the desktops so an app should utilize them where they are useful. For example your website provides information about a chain/list of restaurants. In most cases users are interested about such information regarding restaurants nearer to their current location. So instead to displaying all the results and then asking your users to select their location from dropdown or other way, your app could detect the users current location and display the results those are more relevant to them highlighted.
An app also reduces user efforts. For example if your site needs the users to log in to access some of its features, an app may eliminate this requirement. That is the users will not require to login each time they require to access those features, yet their account will be secured.
To access your website from browser will take two to three more clicks than that of an app. And in some cases the users need to type the entire url. In the highly impatient society that we live in today, users may opt to choose the effortless way. Also once your users have your app installed on their devices, you already have an advantage over your competitor. Since your user
will not tend to switch from your app to your competitors site that often. Also since unlike a website that runs on a remote server, an app runs on local device so it is more powerful and faster than a website.
Your app may also offer some offline features to your users if possible and that will add points to the users feel good about your app. In case they do not have network connection due to some reasons it will be very helpful for them.
Another important thing to decide before your app development starts is your color code. Once the color code for an app is decided, each and every screen should follow it to maintain the same look and feel for the entire app. Mostly color code for an app is taken from the colors of the app logo. The app icon should have the feel from the site logo and yet it should be attractive enough that it attracts the users to touch it.
Hope this discussion brings some light on what you should know to build and effective app.