Important Features

Sounds
Sometimes referred to as "tones", "voices" or "patches", the number of sounds in an instrument can vary widely. Obviously, you will spend more for more sounds, but the quality of the sounds can also affect price. While you may find a $129.00 keyboard with 100 sounds you can also spend $1099.00 for a unit with only a dozen sounds. Certainly the quality of those dozen sounds will be superior. Your purchasing decision should depend on which features are most important to you.
Polyphony
This is the number of notes that can be played at one time. A polyphonic synthesizer can play more than one note at a time, while a monophonic synth can only play one note at a time. Most modern synthesizers are polyphonic.
Action
The way it feels when you push down on a key. Most Portable keyboards have very light "spring" or "organ touch" action, as opposed to the heavier "weighted action" of a digital piano.
Touch Sensitivity
Sometimes referred to as "Velocity Sensitivity"", this is a feature that allows the degree of loudness in which a note is reproduced, to be relative to the intensity in which the keyboard is struck. In other words, the harder you hit the key, the louder the sound.
Sustain Pedal
An electronic sustain pedal recreates the effect of the mechanical sustain foot pedal found on the traditional piano. When you step on the sustain footswitch, it allows notes to continue to sound after your fingers have left the keyboard.
Sequencer
A built-in Sequencer records "events" that occur as you play the keyboard. It's like having a built-in multi-track recorder. The sequencer remembers what keys you pressed and how long and how hard you pressed them. This is a great tool for recording song ideas or playing back lessons to monitor your progress.
USB Connectivity
Short for "Universal Serial Bus", USB is that slim little rectangular input on your PC. It is an interface standard for hooking peripherals (in this case, your keyboard) to your computer. This allows your keyboard to interact with educational software, programs for recording, sequencing etc.

Keyboard Amps

A Keyboard amplifier is basically a small PA disguised as a combo amp. Modern electronic keyboards are probably the most demanding instruments on the planet. They produce a full range of sounds that are very demanding when it comes to amplifier components. Keyboard amps are required to recreate many different types of sounds with clarity and accuracy. To fully appreciate the quality of the sounds in your keyboard, it's critical that you're amplification system is up to the task.

Solid-state technology is perfect for such a challenge. Solid-state "combos" are amplifiers that combine the speakers and the amplifier in one box.

Many keyboard amps have multiple input channels built into them to accommodate a number of keyboards at one time.

It is not uncommon for the channels on this type of amp to have little or no EQ (equalization) to adjust tone. The sounds produced by today's keyboards are very smooth, well-balanced tones that often sound best without any additional equalization

Keyboard combos usually incorporate a high-frequency driver, also known as a "tweeter", to deliver the crisp high-end frequencies that aren't quite as present with a cone speaker. Many keyboard amps often have multiple sizes of speakers (similar to PA enclosures) to deliver a full-range of crystal clear sound. Many larger keyboard amps can be used as a small PA, with low-impedance inputs for vocal mics. These are often fine for small coffee house or Bookstore gigs, but when the venue is bigger and noisier, it would be best to go with a traditional PA system and use the amp as a stage monitor. Keyboard amps will often feature a low-impedance "line out" which allows you to send the amps mixed signals directly into a PA system.