A patent is a government-granted monopoly right.  The owner of a patent is the only person who can use, make, import, offer for sale, or sell a device or method that incorporates a part or a function described in the patent's claims.  Anybody else who does those things, except under a license from the patent owner, may be liable for patent infringement.

There are two distinct branches of patent practice: prosecution and litigation.  You might not guess it from the name, but "prosecution" is actually what you do to get a patent in the first place.  If you're an inventor, or a business owner and employer of a creative engineer, you may have occasion to seek a patent on a new and useful idea.  An inventor can prepare and prosecute his own patent application, but this area of law is quite complex, and a misplaced word can mean the difference between a strong, valuable patent claim, and a worthless patent certificate (although the certificate looks impressive if you frame it and hang it on the wall of your cubicle).

If your invention is important to your business, entrust the drafting and prosecution to an experienced attorney with a strong engineering background.  You're more likely to end up with claims that you can license profitably, or a patent that you can use to keep competitors out of your market segment.

Patent litigation can come into your life from a couple of different angles.  First, if you are a patent owner, you may want to use your patent to stop a competitor from knocking off your invention.  A patent infringement lawsuit, or at least the threat of a lawsuit, is how you'd do it.  And second, someone else who owns a patent may allege that your product or service infringes his patent, and demand that you take a license, or cease manufacturing or selling the product.

Litigation is closer to traditional legal work, but patent infringement analysis is still a tricky area of law that demands both legal and technical expertise.  The stakes in patent litigation are often very high: a patent owner may be able to shut down your business, or stop the production and sale of a whole product line.  If you receive a Cease & Desist letter alleging patent infringement, or if you're served with a summons and complaint, contact an experienced patent attorney right away.  The consequences of delay can be catastrophic.