Global Level

The Flamingo skin (and some other skins) don't show vertical borders by default. If you want to change that you can add a  StyleSheet Extension (SSX) xobject (i.e. XWiki.StyleSheetExtension) to any wiki page of your choosing (you can create a new page just for this for example), with the following content:

.main th, .main td, .wiki-table th, .wiki-table td { 
    border: 1px solid #E8E8E8; 
    padding: 3px 4px; 
} 

You could also decide to have some background colors for table headers by adding:

th { 
    background-color: #F3F3F3; 
    font-weight: 700; 
}

And if you don't want the table to take the full width you could add:

table { 
  width: auto; 
}

Table Level

It's also possible to say that you want border just for a specific table using the table-bordered CSS class. For example in XWiki Syntax 2.0+:

(%class="table-bordered"%)
|A|B| 
|C|D| 

And if you want a hover effect (you can combine it with borders too):

(%class="table-hover"%)
|A|B|
|C|D|

More generally all the Bootstrap styles can be used.