Meaning of the Tables in the Blockchain Database
I've worked with Abe for some time; I'll try to have a run at it... I'm using the latest table list from Database version Abe39 on MySQL.

Utility tables:


abe_lock:

Used for locking while performing DDL
configvar:

Configuration and results of some auto-detected SQL features.


Blockchain tables:


block:

Block hash and related statistic/infos.
block_next:

Forward-linking of blocks (blocks references only the previous block; this table links to the next one).
block_tx:

Links blocks to all their transactions in tx.
block_txin:

I believe this is merely a "shortcut" table that contains information accessible elsewhere: somewhat links together block_id, txin and the block it's spent on.
orphan_block:

Used to link orphan block (for eventual out of order/multi-threaded indexing?) References the previous block hash (since we do not have pref block_id yet).
tx:

A transaction: links to inputs (txin) and outputs (txout)
txin:

A single transaction input.
unlinked_txin:

As for orphan_block, links to txin we do not have loaded yet. References the txout_hash as we do not have yet the txout_id.
txout:

A single tx output
pubkey:

A bitcoin address (pubkey)
multisig_pubkey:

A multisig pubkey
chain:

Chain name and properties (one per cryptocoin)
magic           

Per-chain magic number
chain_candidate:

Identifies each block height and whenever it's part of the longest chain or not.
datadir:

Information relative to blockfile indexer


Sequences:

This is pretty much DB-specific; on MySQL these are tables that have a single AUTO_INCREMENT column and are used to create unique keys for their respective tables:


block_seq
chain_seq
datadir_seq
pubkey_seq
tx_seq
txin_seq
txout_seq