For example, let's say that you know of a verse that says our hairs are numbered. You could look up the word "numbered" in a Strong's Concordance and it would give you a listing of all the verses that contain the word "numbered". You would then find Matthew 10:30, where Yahushua said that "the very hairs of your head are all numbered". You can find the Strong's Concordance in most any bible bookstore (See the graphic to your right).
Also beside each verse reference there is a number. That number represents a Hebrew word (if in the Old Testament) or Greek word (if in the New Testament). In the back of the book it lists Hebrew and Greek words used to translate the bible into English. Each has a a number beside them so that we may only need to know the number to locate a Greek or Hebrew word. Then we can do a word study by reading the meaning of the original word. Whenever I refer to a number in the Strong's concordance, you can look up the number for yourself in the Strong's Lexicon or other lexicons that use Strong's numbers to verify everything.
One thing to keep in mind is that while the Strong's Concordance is fairly reliable in its lexicon definitions, it is relying on 19th century scholarship. One of the best ways to determine the true meaning of a word is look up that word in a Hebrew or Greek Lexicon to see how it was translated in various places (See below). Also, Hebrew especially has various verb forms, tenses and stems that can have different meanings. The Strong's Lexicon doesn't do much to address this, but others (such as the Brown Driver Briggs that the online concordance uses) have more detailed definitions for each verb stem.
Differences with the Online Concordance
With the online concordance, you have a tool even better than a normal Strong's Concordance. You can search multiple words and find all the verses with those words in it. For instance, you could search using the two words "beginning created" and find 4 verses that contain both of these words. This is something that a normal Strong's Concordance cannot do. Also, you can search the concordance by Strong's number. For instance, if you wanted to find all the occurrences of the Hebrew word #07723 (Shav), you put 7723 in the search box and it would list the scripture verses that contain this word. This makes it possible to get a fuller meaning of the word when you see how it is used in the context of other verses.