Generic Lisp is actually defined in the ANSI standard. For him, there are many implementations, both commercial and FOSS. Implementations tend to conform to the standard and provide some extensions for things that are not defined in the standard (for example, streams, FFI, and Unicode).
You should see http://www.cliki.net/Common%20Lisp%20implementation to compare implementations. Which one you should use depends on your needs and preferences. For starters, you can simply choose one. The language itself will be the same regardless, and until you use any specific extensions for implementation, you can easily switch to another if necessary.
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