What is a "fluid" column in Display Suite?
In Display Suite, the "fluid" refers to the templates ability to expand the column width if one (or more) columns are empty. 

For a 2 column layout, the CSS is:

.ds-2col &gt; .group-left {
  float: left; /* LTR */
  width: 50%;
}

.ds-2col &gt; .group-right {
  float: left; /* LTR */
  width: 50%;
}


So even if there is no content in .group-right, the .group-left element will still take up only 50% of the available width.

For the fluid 2col layout, the CSS is:

.ds-2col-fluid &gt; .group-left {
  float: left; /* LTR */
  width: 50%;
}

.ds-2col-fluid &gt; .group-right {
  float: right; /* LTR */
  width: 50%;
}

.ds-2col-fluid.group-one-column &gt; .group-left,
.ds-2col-fluid.group-one-column &gt; .group-right {
  width: 100%;
  float: none;
}


The last ruleset (the one with '.group-one-column') means that if either of the columns contains no content then the other column will expand to 100% of the available width. You can see the little piece of PHP logic that adds the extra CSS class to the template here: http://drupalcode.org/project/ds.git/blob/refs/heads/7.x-2.x:/layouts/ds_2col_fluid/ds-2col-fluid.tpl.php

For your case, either is fine, but to get the 25% / 75% split, you'll need to override the DS CSS code in your own theme's stylesheet.

You could use something like

body .ds-2col .group-left {
    width: 25%;
}  
body .ds-2col .group-right {
    width: 75%;
}