exact keyword means same keyword which you suggest and broad keywords are those keywords which are heaving some extra words/keywords in exact keywords. suppose you can find the keyword like "Web design" in googleadwords then it suggest some other keywords similar to main keyword. for eg. Website Design, Web Design company, Free web design etc.
Broad Match: The keywords using the term broad match can be shown in any order with any other words. For example. The term ‘water feature’ could be shown with any variation. i.e.: water pump feature, water house feature, feature water house, feature hose water. Phrase Match: The keywords are used in the order shown with other words for variation. For example: The term ‘water feature’ could be shown with another word accompanying this term. i.e.: gold water feature, water feature blue, bespoke water feature, elegant water feature.
Broad match = Contains all the words, may be in any order, and may include other words Exact match = Contains only these words in this order
The name says it all that the exact will give you the exact keywords and the broad will give us the relevant keywords also.
Exact will show details on the exact same word or phrase that you've searched for. And broad will show you related stuff as well.
Exact is used to find out exact number of searches for the keyword and broad includes all relevant keywords