Overstrung  - the bass strings (usually about 27 notes) cross over the treble ones. This allows for greater length in the bass and tenor area without increasing the height, and also means that the bass and tenor bridge is nearer the vibrating part of the soundboard, making the tone richer. However there are some exceptions and some top firms like Bechstein, Bluthner and Lipp deliberately produced straignt strung pianos as this form of stringing aviods a diffence of tone at the cross over point, better damping and a sweeter tone. 

Underdamped  - The dampers are underneath the hammers and rely on springs to hold them against the strings. This system is not as efficient as that on a grand piano where the dampers are directly above the hammers and therefore dampen at the main vibrating point, and they also rely on gravity. Provided they are well regulated they dampen more efficiently and the efficiency does not decrease with wear as with the upright springs.
Top makes of older upright pianos are also underdamped, with the exception of some Bluthner and Ibach uprights which while overdamped are still excellent pianos. Perhaps 50% of other pre 1920 pianos are overdamped, and while the tone might be excellent they usually shut off the sound less efficiently, especially in the mid to upper treble (the top two octaves are not dampened in any piano as the notes don't vibrate for long.

88 keys - Pianos after about 1885 usually have either 85 or 88 keys. The top three keys, Bb, B & C, are very rarely used (perhaps one piece in a thousand!) and exam study can be done on any 85 note piano. There are some pianos with 6 octaves (F to F). All composers to Mozart and most of Beethoven can be performed on these, though the bass tone is especially thin owing to the short string length. 

Iron frame -  Practically all pianos from about 1900 have full iron frames. Before this there are quite a few uprights with half-plate frames and earlier still, no frame at all. Nearly all of these are also overdampers. Not many uprights with no frame are still being used, though we occasionally come across some wooden framed English pianos by makes such as Broadwood.

Three pedals - the main pedal used is the right hand or sustain pedal. This lifts the dampers causing all the notes to vibrate in sympathy with the ones played. The left hand pedal on modern upright pianos lifts the hammers nearer the strings which encourages softer playing. The middle pedal is usually a practice pedal, dropping a "celeste" felt between the hammer and the string. This allows very soft playing while practising. Some top quality modren uprights have a sostenuto mechanism instead of the celeste. When the middle pedal is depressed after a note has been played, the damper is held up continuously on that note only. This idea was orininally so that an organ player could sustain a "pedal bass" while using both hands to play the other lines of music.