What's the difference between HSDPA and HSUPA?

HSDPA improved the downlink (tower to phone), while HSUPA improved the uplink (phone to tower).

HSDPA (High Speed Downlink Packet Access), 2005, "3.5G":

• Focused on download speed, the direction that matters most for browsing, streaming, and downloads.
• Used adaptive modulation (16QAM): the phone and tower negotiate the fastest modulation the signal quality allows.
• Introduced a high-speed MAC protocol to reduce latency.
• More efficient use of the radio channel.
• Speeds: down 14.4 Mbit/s.

HSUPA (High Speed Uplink Packet Access), 2007:

• Fixed the bottleneck in the other direction: UMTS uplinks were painfully slow.
• Theoretical max: 5.76 Mbit/s upload.
• Made uploading photos, sending emails with attachments, and video calls much more practical.

HSPA+ (evolved HSPA):

• Further upgrade reaching up 11.5 Mbit/s, down 42 Mbit/s.
• Made mobile internet genuinely usable for everyday tasks.

The pattern: Each "point" generation (3.5G, 3.75G) squeezes more performance out of existing infrastructure before a full generational leap requires new architecture.

Go deeper:

• High Speed Packet Access (Wikipedia) — the combined HSPA family, with separate sections for the HSDPA downlink, HSUPA uplink, and evolved HSPA+.