My firm is regularly asked to render legal opinions on issues related to patents.  There are several different types of patent opinions: 

Patentability Opinion. 
A patentability opinion is typically requested when a client wants to learn the likelihood of obtaining a patent on a new device, method, composition, etc.  It is usually prepared before a decision is made to proceed with a patent application.  A patentability opinion is based on a search for prior art.

Right-to-Use Opinion. 
A right-to-use opinion is usually requested when a client wants to know whether that client’s new product or process infringes any active patents.  In other words, the client wants to know whether its new product or process will potentially expose the client to a lawsuit for patent infringement.  This opinion may help the client decide whether or not to move forward with the new product or process.  A right-to-use opinion is based on a search for active patents.

Non-Infringement Opinion. 
A non-infringement opinion is frequently requested when a client wants assurance that it is not infringing a particular patent.  The patent may be owned, for example, by the client’s competitor. 

Infringement Opinion. 
An infringement opinion may be requested by a patent holder when the patent holder wants to know if someone else is infringing the patent holder’s patent.  This opinion may help the patent holder decide whether to file suit for patent infringement.

Validity Opinion. 
A validity opinion is usually requested by a client who is concerned about infringing a particular patent and wants an opinion on the validity and enforceability of that patent.  A validity opinion may also be requested by a patent holder that is considering a lawsuit against an infringer.  The patent holder may want assurance that its patent is valid and enforceable before taking on the costs of enforcing its patent.