Some stamps were/are issued in full sheets, others in strips (called coils) without perforations on opposing sides and some were issued in booklets where some stamps may be straight edged on one or two sides. Others were just poorly perforated. It's a learning process.
Top Row 1. orange is called a pre-cancel - 2 parallel lines with town & state preprinted between the lines. Call pre-cancel because they were purchased in 'cancelled' condition from the post office by the user. Generally used by a company for bulk mailings.
2. green with 2 straight edges is a coil stamp
3. Green 4 perf sides is a regular issue stamp
4. Green 4 perf sides is a regular issue stamp, but appears to have a 'clipped perf' in upper right hand corner
Bottom Row
1. Red with 3 perfed sides and one straight edge is a regular issue from the bottom edge of a sheet hence the straight edge (no perfs) - also this one has 'holes' punched in the stamp. This is call a 'perforated' stamp as the holes were punch by a company in their stamp stock to prevent theft and misuse of the stamps.
4. Purple, straight edge on this stamp is a result of being cut off by someone. It originally had 4 perfed sides.
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