Plz guide me about CSS Classes.

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CSS

I am a new comer in the field of CSS. But I have a confusion about CSS Classes.
For Example

#logo p {
    float: left;
    margin: 0;
    padding: 26px 0 0 10px;
    font: normal 14px Georgia, "Times New Roman", Times, serif;
    font-style: italic;
    color: #5E4E38;
}
#logo p a {
    color: #5E4E38;
}
#logo a {
    border: none;
    background: none;
    text-decoration: none;
    color: #A83A01;
}

Here I want to understand that Why #Logo p, #Logo p a, #Logo a and #Logo p etc has been used. We can use only #Logo but what are these (a, p, p a etc) with #Logo CSS Class. What is their functionality?