The most common type of outdoor speaker is the bracketed box speaker, basically a weatherproof version of a traditional speaker design. They can be mounted to buildings, trees, poles - pretty much anything. They sound good and many can be painted to match their envirionment.
There are also speakers that double as planters. They are somewhat convincing looking depending on your idea of what a planter should look like. This is somewhat a niche product and there is not a lot of variety. They generally fire a speaker down towards a reflecting panel producing a more diffuse sound than traditional front-firing speakers. Their 360 degree dispersion pattern makes them very good for background music but not the top choice if you are looking for the best fidelity.
Landscape speakers are designed to be placed in your plant beds. There are full range models and satellite systems that resemble outdoor lights and come in a variety of sizes and configurations. They can cover large areas and certain types can be tied to long wire runs. The speaker enclosures are relatively small so most include subwoofers or have subs are available. They are really necassary for this type of solution if you want the best sound. Learn more...