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I found a symbol that I am interested in in the Comprehensive LaTeX Symbols list, where it is noted that the symbol is part of the ifsym package.

However, when I try to use the symbol, thus:

documentclassarticle
usepackageifsym
begindocument
[
a SmallSquare b
]
enddocument

the document fails to compile, and I get the following error message:

ERROR: Undefined control sequence.

--- TeX said ---
l.5 a SmallSquare
 b

EDIT: It was suggested in a comment that I load the package with the geometry option. So I tried it (usepackage[geometry]ifsym). The good news is that now the document compiles successfully. The bad news is that the symbol that gets printed is wrong:

By default the ifsym package defines only a generic textifsymbol macro. Specific abbreviations are declared in different subsets which can be loaded by the options clock, weather, alpine, misc, geometry, electronic. The geometric shapes must be loaded by

documentclassarticle
usepackage[geometry]ifsym
usepackageamsmath % for correct scaling in sub/superscripts
begindocument
[
a textnormalSmallSquare b
]
enddocument

The Comprehensive Symbol List lacks this detail.

Note that the symbols are text mode, so they will fail in math mode. As an example, SmallSquare expands more or less to

fontencodingUfontfamilyifgeoselectfont symbol96

The font switching stuff is useless in math mode and symbol96 is all is left, leaving the @.